Electrophoretic display medium and device

ABSTRACT

An electrophoretic display medium includes at least one set of colored particles in a dielectric fluid, wherein the display medium has an electrical conductivity of about 10 −11  to about 10 −15  S/m. The display medium is included in an electrophoretic display device by including the medium in a multiplicity of individual reservoirs of a display layer or layers that is located between conductive substrates.

BACKGROUND

Described herein is an electrophoretic display device. Moreparticularly, described is an electrophoretic display device containingcolorant particles capable of field-induced charging. Theelectrophoretic display devices herein are capable of generating images,including full color images. The electrophoretic displays herein may beused for any display application, and particularly any displayapplication where the image displayed may be changed, including, forexample, reimageable paper, electronic books, electronic signage, watch,monitor and/or cell phone displays, and the like.

One advantage of field-induced charging is that the colored particles ofthe display nay be made to more rapidly and reliably respond to anelectric field application in displaying an image, potentially with muchlower energy costs. This allows for the electrophoretic display deviceto be used in displays requiring rapid image switching capabilities, forexample such as monitors.

Electrophoretic displays are well known in the art. An electrophoreticdisplay generally comprises a suspension of one or two charged pigmentparticles colloidally dispersed in a clear or colored liquid of matchingspecific gravity and contained in a cell comprising two parallel andtransparent conducting electrode panels. The charged particles aretransported between the electrode panels under the influence of anelectric field, and can therefore be made to display an image throughappropriate application of the electric field on the electrodes. Theadvantages of electrophoretic displays as a means for providinginformation and displaying images has been well appreciated.

U.S. Pat. No. 4,272,596, incorporated herein by reference in itsentirety, illustrates an electrophoretic display structure. Theelectrophoretic display device comprises a white marking material suchas titanium dioxide suspended in a colloidal dispersion containing blackcolorant such as colloidal size iron oxide particles known asferrofluids. Image formation is achieved electrophoretically byselective application of an electric field across the imagingsuspension. In particular, a pair of electrodes associated so as to forma cavity therebetween, which cavity is filled with the aforementionedsuspension medium. A source of electrical potential is coupled to theelectrodes and when an electric field is applied, the marking particlesform an image as they follow the field.

U.S. Pat. No. 6,113,810, incorporated herein by reference in itsentirety, describes a dielectric dispersion for use in anelectrophoretic display that includes a dielectric fluid, a firstplurality of particles of a first color having a surface charge of aselected polarity dispersed within the dielectric fluid and a secondplurality of particles of a second color having a surface charge ofopposite polarity to that of the first plurality and a steric repulsionthereto preventing coagulation of the first and second plurality ofparticles. Each set of particles is formed with unique secondary andfunctional monomers. Corresponding charge control agents are added tothe dispersion to establish opposite polarities on the respectiveparticles.

U.S. Pat. No. 6,017,584, incorporated herein by reference in itsentirety, discloses electrophoretic displays and materials useful infabricating such displays. In particular, encapsulated displays aredisclosed in which particles encapsulated therein are dispersed within asuspending, or electrophoretic, fluid. This fluid may be a mixture oftwo or more fluids or may be a single fluid. The displays may furthercomprise particles dispersed in a suspending fluid, wherein theparticles contain a liquid. In either case, the suspending fluid mayhave a density or refractive index substantially matched to that of theparticles dispersed therein. Application of electric fields to theelectrophoretic displays affects an optical property of the display.

U.S. Pat. No. 6,577,433, incorporated herein by reference in itsentirety, discloses an electrophoretic display liquid composition foruse in an electrophoretic display device that has a multiplicity ofindividual reservoirs each containing the display liquid of two sets ofparticles dispersed in a transparent liquid system as well as at leastone charge director dissolved or dispersed in the liquid system, orphysically embedded on the surface of the particles or chemically bondedon the surface of the surface of the particles, the two sets ofparticles exhibiting different, contrasting color and different chargingproperties from each other. The charge director(s) may include a metalsalicylate compound. The particles may be modified with chargecontrolling agents, and may also include a set of magnetic particles.The transparent liquid system may include two immiscible liquids havingdifferent densities with the sets of particles having densities inbetween the densities of the two immiscible liquids such that theparticles rest at an interface between the two immiscible liquids.

U.S. Pat. No. 6,525,866, incorporated herein by reference in itsentirety, discloses an electrophoretic display liquid composition foruse in an electrophoretic display device that has a multiplicity ofindividual reservoirs, each containing the display liquid of at leasttwo sets of particles dispersed in a transparent liquid system, the atleast two sets of particles exhibiting different, contrasting color anddifferent charging properties from each other, and at least one of thesets of particles containing flow aid particles as additives upon anexternal surface of the particles. Preferred flow aid additives includesilica and titania particles.

Electrophoretic display is thus based on the migration of chargedparticles suspended in an insulating fluid under the influence of anelectric field. The particles used in such displays to date have beencharged by adding a charge control agent, which is capable of ionicdissociation, to the dielectric fluid during preparation of thenon-aqueous display dispersion. Examples of charge control agents usedhave included bis-(2-ethyl hexyl) sodium sulfosuccinate and basic bariumpetronate (BBP). Dissociation of the charge control agent into positiveand negative ionic species in the dielectric fluid results inpreferential surface absorption of ions of one polarity by theparticles. The particles therefore become charged. The resultingdispersion contains a complex mixture of particles including chargedparticles, excess free ions and counter-ions. Due to the presence ofexcess free ions, such electrophoretic display inks are characterized byhigh electrical conductivity. Conductivity has been shown to increasewith concentration of the added charge control agent, and is typically100-1000 times higher compared to the dielectric fluid. Highconductivity of the ink results in increased power consumption andslower switching speed of the display.

While known electrophoretic display devices, compositions and processesfor displaying images with such known devices are suitable for theirintended purposes a need remains for an electrophoretic display thatremains stable for long periods of time and that reliably and rapidlydisplays and/or changes an image, and in particular a full color image.

SUMMARY

In embodiments, described is an electrophoretic display medium,comprising at least one set of colored particles in a dielectric fluid,and wherein the display medium has an electrical conductivity of about10⁻¹¹ to about 10⁻¹⁵ S/m.

In further embodiments, described is an electrophoretic display device,comprising a multiplicity of individual reservoirs containing a displaymedium between conductive substrates, at least one of which istransparent, wherein the display medium comprises at least one set ofcolored particles in a dielectric fluid, and wherein the display mediumhas an electrical conductivity of about 10⁻¹¹ to about 10⁻¹⁵ S/m.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of an electrophoretic display device.

FIGS. 2-11 illustrate a process of making a flexible electrophoreticdisplay device in which the display layer comprises a grid patternformed on a conductive substrate to define individual cells each filledwith display medium. FIGS. 2-6 illustrate steps to form the grid patternon the substrate and FIGS. 7-11 illustrate filling the individual cellsand bonding to form the display device.

FIG. 12 illustrates a flexible electrophoretic display device.

FIG. 13 illustrates another embodiment of an electrophoretic displaydevice.

FIGS. 14 and 15, in which FIG. 15 in an inset of FIG. 14, illustrate adisplay layer having a multiplicity of cavities filled with displaymedium.

FIG. 16 illustrates a display device including a color filter.

FIG. 17 illustrates a device for charging particles of a display device.

FIGS. 18 to 23 illustrate charging characteristics of particles for usein electrophoretic display devices.

FIGS. 24 to 27 illustrate methods of controlling the color displayed bya cell of a display device.

EMBODIMENTS

Display Device Structures

Structures of electrophoretic display devices in which a display mediummay be included will first be described. Use of the electrophoreticdisplay mediums described herein is not, however, necessarily limited tothese embodiments, and any other suitable design for an electrophoreticdisplay device may be used without limitation. As an example of asuitable electrophoretic display device design not specificallydescribed herein that may nevertheless be used with the present displaymedium, U.S. Pat. No. 6,788,449, incorporated herein by reference in itsentirety, is identified.

As illustrated in FIG. 1, an embodiment of an electrophoretic displaydevice comprising two conductive substrates 10 and 20 disposedoppositely of each other, with an electrophoretic or display layer 40therebetween. The display layer may have a thickness of from, forexample, about 5 to about 1,000 μm, such as from about 10 to about 500μm or from about 20 to about 350 μm.

Layer 40 may be comprised of a layer that includes spacers therein,which spacers define a multiplicity of individual reservoirs that eachcontain the display medium (30, 31 and 32) comprised of fluid andcolored particles. A multiplicity refers to, for example, from about 2to about 100,000,000, or potentially more, such as from about 100 toabout 50,000,000 or from about 1,000 to about 1,000,000. Thus, forexample, if each of the multiplicity of reservoirs is about 100 micronsacross, a square of 1,000×1,000 reservoirs (or about a 4 inch×4 inchdisplay) would have about 1,000,000 total reservoirs. In this regard,each reservoir may be thought to correspond to a pixel of the device.Reservoir refers to, for example, any unit containing, or capable ofcontaining, display medium therein, and includes, for example, unitsseparated by a spacer device, pockets, cavities or bubbles formed in asingle sheet or between two sheets, capsules or microcapsules in a sheetor layer, and the like.

In the FIG. 1 embodiment, the particles are shown to include a set ofblack particles and a set of white particles. However, as will bediscussed more fully below, the particles may be comprised of at leastone or multiple differently colored particle sets, for example from 1 toabout 10 particles sets, such as from 1 to about 6 particle sets or fromabout 2 to about 4 particle sets.

As the conductive substrates of the electrophoretic display device, anysuitable materials may be used without limitation, for example includingmaterials presently known and used or that may be used in the future inthe art. At least one of the conductive substrates, in particular atleast the top conductive substrate through which the images formed bythe device may be viewed, should be transparent in order to enable suchviewing. Both substrates may be transparent, if desired. The bottom orback substrate need not be transparent, and may instead be, for example,a light reflecting or light absorbing material. As suitable materialsthat may be used, mention may be made of conductive polymer films, forexample polymer films coated with a transparent conductive material suchas indium tin oxide (ITO), such as polyethylene terephthalate (PET)films, for example MYLAR (Du Pont), polyethylene napthalate (PEN) films,polyethersulfone (PES) films and the like, conductive glass films, suchas ITO coated glass, and conductive thin metals. For transparency, ITOcoated polymer films and glass are suitable. The substrates may eitherbe flexible or rigid.

The substrates that sandwich the spacer layer therebetween may have alength and width corresponding to the overall length and width of theelectrophoretic display device. The substrates thus may be continuous,unitary films that are not present as just separated pieces over justindividual reservoirs of the display device, although a plurality ofsegregated substrates may also be used. The substrates may be made to beas thin as possible while still maintaining appropriate conductiveproperties and structural integrity. For example, the substrates mayhave a height, or thickness, of from about 10 microns to about 500microns, such as from about 10 to about 250 microns or from about 20 toabout 100 microns.

Between the conductive substrates are contained a multiplicity ofindividual reservoirs (30, 31, 32), each filled with a display mediumdescribed more fully below. Each of the individual reservoirs definesone container and/or cell of the electrophoretic display device.

In embodiments, spacers may be used to keep the individual reservoirsseparate from one another. Any suitable spacer design may be used. Forexample, the spacer may be of the type described in U.S. Pat. No.7,123,238, incorporated herein by reference in its entirety. The widthand/or diameter of the individual reservoirs may be from, for example,about 5 microns to about 400 microns, such as from about 5 to about 200microns or from about 5 to about 50 microns. Also, the spacer layer 40may be comprised of more than one layer/sheet, such as from two to abouteight layers or from about two to about four layers, for example whenpocket sheets having differently colored display mediums therein arestacked together.

The display medium to be used within the reservoirs contains particlesof a size smaller than the reservoir width/diameter in order tofunction.

Where the spacer layer is comprised of a multiplicity of individualreservoirs, a solid portion of the spacer separating the multiplicity ofreservoirs, that is, the spacing or partition between individualreservoirs of the spacer layer, are desirably as thin as possible.Preferred spacing/partition thicknesses are on the order of, forexample, about 10 microns to about 100 microns, such as from about 10microns to about 75 microns or from about 15 to about 50 microns.

The display device may have any suitable overall length and width asdesired. The electrophoretic display device may also be made to have anydesired height, although a total height of from about 30 to about 1,000microns, such as from about 30 to about 400 microns or from about 50 toabout 300 microns, may be used in terms of size and ease of use of thedevice.

In forming the electrophoretic display device, the reservoirs, forexample pockets, of the spacer layer are filled with the display mediumand the spacer layer is located over a first, or bottom, conductivesubstrate. The filling of the reservoirs and location of the spacer overthe substrate may be done in any suitable order. In embodiments, thespacer layer may be physically attached to the first conductivesubstrate or intermediate films, which may be done by any suitablemethod. Adhesive may be used for convenience, although other attachmentmethods such as sputtering deposition of the conductive film may also beused. Once the reservoirs are filled with display medium and the spaceris located over the first conductive substrate, the second, or top,conductive substrate, is located over the spacer layer. In non-pocketreservoirs and/or in displays not including any intermediate layers,this may act to seal the reservoirs. The first and second substrates mayalso be located in association with the spacer layer in reverse order,if desired, and may also be associated with the spacer layer at the sametime, for example where the spacer layer comprises a sheet ofindividually enclosed pockets filled with display medium. Again, thelocating of the second conductive substrate in association with thespacer layer may be done by attachment, if desired, by any suitablemeans, including gluing with an adhesive. Additional intermediate layersmay be included between the spacer layer and conductive substrates asdesired, and thus the location and/or attachment as described above neednot be a direct attachment or association of the spacer to theconductive substrates.

In embodiments, the display device may be made to be flexible. In thisembodiment, the substrates are each comprised of a flexible polymericfilm, and the spacer comprises a grid pattern on at least one of thesubstrates. The grid pattern may be integral with one or both of thepolymeric film substrates. Integral refers to, for example, the gridpattern walls or sidewalls that segregate the individual cells of thedisplay device being comprised of the same material as the polymericfilm substrate and being formed with the polymeric film in the samemolding step. For flexibility, each film may have a thickness of fromabout 5 to about 75 μm, for example from about 10 to about 50 μm or fromabout 10 to about 30 μm. The overall device including joined films mayhave a thickness of less than 150 μm, for example from about 10 to about150 μm or from about 20 to about 75 μm.

The width and/or length of the individual reservoirs of the grid patternare preferably from, for example, about 5 microns to about 200 microns,such as from about 5 to about 100 microns or from about 10 to about 100microns. Obviously, the display medium to be used within the reservoirsmust contain particles of a size smaller than the reservoir width/lengthin order for the display to function. The solid portion, that is thewalls, of the grid separating the multiplicity of reservoirs, aredesirably as thin as possible. Partition thicknesses on the order of,for example, about 10 microns to about 100 microns, for example about 15to about 50 microns, may be used.

The film with a grid pattern formed thereon has the cells defined by thegrid walls filled with display medium, and then the displaymedium-containing film is joined to another flexible polymeric filmsubstrate, for example a film without a grid pattern thereon or a filmitself having a grid pattern and also filled with the same displaymedium. The joining may be achieved by any method, for example heatsealing and/or with the use of an adhesive. If an adhesive is used, theadhesive may have a repulsive interaction with the display medium sothat the display medium is retained in the cells of the grid duringjoining. For example, if the display medium is hydrophobic an adhesivehaving hydrophilic characteristics may be used.

To form the flexible polymeric film having the grid pattern formedthereon, a master for molding (micromolding) is first prepared. This maybe done by any suitable technique, for example through appropriateexposure (for example through a photomask) and development of aphotoresist material film such as SU-8 (a commercially available(Microchem Corp.) spun-on epoxy) located on a substrate, for exampleglass. Additional suitable materials and microfabrication techniques forforming a master may also be used, for example including etching into asilicon or glass or fabricating by electroplating or electrolessplating. U.S. Patent Publication No. 2005/0239935, incorporated hereinby reference in its entirety, describes methods and materials for themolding steps. The developed pattern corresponds to the desired gridpattern of the flexible film substrate.

In addition, the surface of the master may be coated with a low surfaceenergy coating or a release layer. Examples include fluoropolymers suchas TEFLON AF (DuPont), CYTOP (Asahi Glass), long-chain fluorinatedalkylchlorosilanes, mixtures thereof and the like.

A reverse image master stamp is then prepared, which master stamp isused in forming the final flexible polymeric film with the grid patternformed therewith and thereon. To produce the master stamp from themaster, a material having good release properties, for example asilicone material such as PDMS (polydimethylsiloxane) (available asSYLGARD 184 from Dow Corning) may be used. Other materials for themaster stamp/mold that may be used include, for example, any polymerhaving, or treated to have, suitable release properties, for exampleincluding UV curable polymers, or a metal mold, for example nickel,which enables the lifetime of the mold to be longer. The mold may becoated with a release agent such as a fluorocarbon (for example CYTOP),a low surface energy silane (for example, OTS or a fluorosilane) or asilicone. Commercially available release agents such as Taylor T-WET 630or Taylor T-SIL 50 may be used.

An example process for forming the master stamp is illustrated in FIGS.2-4. To make the master stamp 52, the material thereof, for example asilicone, may be mixed with a curing agent at a ratio of material tocuring agent of, for example, from about 50:1 to about 5:1 such as fromabout 25:1 to about 5:1 or from about 10:1 to about 5:1. Suitable curingagent materials depend upon the material used to make the stamp. Forexample, for SYLGARD 184 PDMS, a suitable curing agent may include amixture containing crosslinker, inhibitor/moderator, and siliconereinforcing resin. Examples of crosslinkers include hydride functionalsiloxane crosslinker material such as HMS-151(methylhydrosiloxane-dimethylsiloxane copolymer), available from Gelest.Examples of inhibitor/moderator includetetramethyltetravinylcyclotetrasiloxane. Examples of siliconereinforcing resin include vinyl “Q” reinforcing resin, a vinylterminated PDMS such as VQM-135 available from Gelest. The mastermicrocell array 50, optionally on a substrate 51 such as glass and thelike, is placed face up in a holder, for example a TEFLON bolder, thataids in releasing the mold after curing. The material for the masterstamp/mold such as silicone is then applied over the cells in a thinlayer (FIG. 2). The mixture may be evacuated to remove any entrappedair. Optionally, remainder of the mixture may be applied over the moldand again evacuated to remove all air bubbles. The material is thencured, for example at about 25° C. to about 300° C., such as from about25° C. to about 250° C. or from about 50° C. to about 200° C., and/orsolidified, and thereafter the master stamp 52 is removed from themaster 50 (FIG. 4).

The flexible polymeric substrate 55 may then be formed from the masterstamp. As the polymer, a substantially clear lower viscosity materialmay be used, for example a material such as a curable, for example UVcurable, adhesive. For example, an epoxy acrylic such as 60-7155 fromEpoxies, Etc., or a urethane acrylic such as 60-7165 (Epoxies, Etc.),may be used. Other materials such as described in U.S. Publication No.2005/0239935 may also find application here. The polymer is not limitedto UV curable polymers; thermoplastic polymers, thermally cross-linkingpolymers or two component reactive systems may also be chosen. A releaseagent, for example such as Duponol WAQ (sodium lauryl sulfate) inisopropanol, Dow Corning 230 fluid (alkylaryl polysiloxane fluid)diluted with chloroethylene, and/or petroleum jelly in a chlorinatedsolvent may be applied to the silicone master stamp 52 to aid inseparation of the cured polymeric film therefrom following molding. Thepolymeric material 55 is applied to the silicone master stamp and/orspread across the surface of a flexible substrate 56 such as ITO coatedMYLAR, and the master stamp is pressed into the polymeric material 55 soas to completely fill the cells of the master stamp 52 (FIG. 5). Thepressure may be uniformly applied, for example through use of a roller.A flat plate may also be placed on the sample and clamped to provideuniform pressure during curing. The sample may then be cured, forexample via exposure to UV light and/or to an elevated temperature, forexample for about 5 to about 60 seconds, such as about 30) seconds,using a DYMAX 5000-EC 400W UV exposure system. The sample may be removedfrom the clamps and cured for an additional amount of time, for examplefor about 5 seconds to about 30 seconds, such as about 10 seconds. Thefilm 55 oil the substrate 56 may then be peeled away from the masterstamp (FIG. 6). The final film with grid pattern may be rinsed, forexample with isopropanol and the like, to remove any residue.

In embodiments, the substrate may be non-flexible, such as glass, ITOcoated glass and the like. In this case, a flat film of the polymer isfirst formed on the rigid substrate, and then peeled therefrom andplaced on a flexible substrate for further processing as above.

The flexible polymer film with the grid pattern thereon may then befilled with display fluid and bonded to form the display device. Thedisplay fluid may be applied across the film to fill the cells of thegrid pattern, and typically excess display fluid is wiped or scraped offof the edges before bonding. It is desirable for the fluid to belocalized in the cells only, and the bonding surfaces clean and free ofresidual fluid.

As an additional step, the bonding surfaces of the film may be modifiedso as to have a lower surface energy than the surface tension of thefluid. In this way, the fluid will not wet the bonding surface. Forexample, by stamping the polymeric film with a low surface energymaterial, for example such as a fluorocarbon polymer, a silane or analkyl chain material of, for example, about 8 to about 1,000 carbonatoms in length, the stamped edges will not be wet by the fluid of thedisplay medium in the cells, ensuring a good bond to another film. Theaforementioned low surface energy materials typically have a surfaceenergy that is lower than the fluid of the display medium, which may be,for example, a silicone fluid or ISOPAR. The coating of the bondingedges may be achieved by, for example as shown in FIGS. 7 and 8,stamping or contacting the top surface of the flexible film 55 with alow surface energy material 58 so as to coat the tops of the grid/cellswith the material. Upon subsequent filling of the cells with displaymedium 60 (FIG. 9), the display medium does not wet the tops of thecells so as to be retained in the cells and so as to keep the topsurface of the cells free of display medium that might interfere withsubsequent bonding of these surfaces.

FIGS. 10 to 12 illustrate an example process for bonding two filledpolymeric films 55 together to create the flexible display device 65containing the display medium in individual cells 61. The adhesionbetween the two films may be strengthened through the use of heat,pressure and/or light exposure. The final flexible device 65 includesindividual cells 61 filled with the display medium as shown in FIGS. 11and 12.

Of course, the foregoing procedure for making flexible film substratescan also be used to similarly make non-flexible display devices. In thisregard, the rigid substrate, for example ITO coated glass and the like,may have the grid pattern formed thereon as in the process for formingthe master discussed above. For example, a photoresist material such asSU-8 and the like may be spun onto the substrate, exposed via aphotomask, and developed to form the grid pattern on the substrate.

Similarly, a photolithographically defined grid pattern may also beformed on a flexible substrate such as a 50 micron thick sheet of MYLAR(which may be coated with a conducting ITO layer). In this case, theflexible substrate may have to be attached to a rigid substrate duringthe processing to ensure flatness during the processing. One way toattach a flexible substrate to a rigid substrate is via a double sidedUV-release adhesive tape such as UC-228W-110 from Furukawa Electric Co,Ltd.

As an example, SU-8-25 (Microchem Corp.) may be spun on the substrate atabout 1,000 to about 3,000 rpm, for example about 2000 rpm, to provide afilm having a thickness of about 10 to about 100 μm such as from about20 to about 50 μm or from about 20 to about 40 μm. The spun on coatingmay be baked, for example on a leveled hotplate, and for example forabout 1 to about 20 minutes, for example about 5 min, at about 80 toabout 150° C., for example at about 115° C. The photoresist is thenexposed to UV light, for example having a wavelength of about 340-400 nmfor about 2 to about 10 min such as about 3 min at 8 mW/cm² through aphotomask. An optional post-exposure bake may be conducted on thehotplate for about 1 to about 20 minutes, for example about 5 min, atabout 80 to about 150° C., for example at about 115° C. The photoresistis then developed in a suitable developer, for example PGMEA (propyleneglycol monomethyl ether acetate, which is a suitable developer for SU-8;other photopolymers may require different developers, as understood inthe art). The developed photoresist film may then be rinsed withisopropanol or the like, and subjected to a final hardbake, for exampleat about 100 to about 250° C. such as about 150° C. for about 1 to about20 minutes, for example for about 5 minutes. Thereafter, a low surfaceenergy surface coating may be applied, for example such as a CYTOPcoating (an amorphous soluble perfluoropolymer film, available fromAsahi Glass Co.). The low surface energy coating forms a nonstick filmto prevent adhesion of particles to the electrode or polymer film. Thecoating may have a thickness of from, for example, about 10 to about1,000 nm, such as from about 50 to about 250 nm or from about 100 toabout 200 nm.

Another embodiment of a suitable electrophoretic display device isillustrated in FIG. 13. In FIG. 13, the electrophoretic display deviceagain comprises conductive substrates 10 and 20 disposed oppositely ofeach other. However, in this embodiment, the layer between thesubstrates is comprised of a multiplicity of microcapsules 45 that haveelectrophoretic display medium encapsulated therein. The microcapsulesmay be held in a suitable matrix material. A similar electrophoreticdisplay device utilizing microcapsules is described in U.S. Pat. No.6,017,584, incorporated herein by reference in its entirety. Themicrocapsules may be made to have a size (diameter) of from, forexample, about 5 microns to about 1,000 microns, such as from about 5 toabout 200 microns or from about 5 to about 50 microns.

In this embodiment, the microcapsules may be prepared and filled withthe display medium, and then the microcapsules are fixed or glued ontoone or both of the conductive substrates, or onto intermediate layersbetween the microcapsules and the substrates, or onto other layers ofmicrocapsules in the device if multiple layers are used. Desirably, themicrocapsules form a monolayer (a layer having a thickness substantiallycorresponding to the average diameter of the microcapsules of thatlayer) in the display layer of the display device. However, multiplelayers, for example 2 to about 10 or 2 to about 4, may also be used.

For making the microcapsules any suitable method of encapsulation may beused. The process of encapsulation may include conventional or complexcoacervation, interfacial polymerization, in-situ polymerization,electrolytic dispersion and cooling, or spray-drying processes. In theseprocesses, the display medium is added to a solution of the wall-formingmaterial to be encapsulated thereby, and the resulting encapsulatedmicrospheres may be subjected to crosslinking. The microcapsules may beprepared using melamine-formaldehyde, urea-formaldehyde,resorcinol-formaldehyde, phenol-formaldehyde, gelatin-formaldehyde,isocyanate-polyol, interpolymer complexes of two oppositely chargedpolymers such as gelatin/gum arabic, gelatin-polyphosphate, andpoly(styrene sulfonic acid)/gelatin, hydroxypropyl cellulose, mixturesand/or combinations of the foregoing, and the like, as microcapsulewall-forming materials.

The interfacial polymerization approach relies on the presence of anoil-soluble monomer in the electrophoretic composition, which is presentas an emulsion in an aqueous phase. The monomers in the minutehydrophobic droplets react with a ionomer introduced into the aqueousphase, polymerizing at the interface between the droplets and thesurrounding aqueous medium and forming shells around the droplets.Although the resulting walls are relatively thin and may be permeable,this process does not require the elevated temperatures characteristicof some other processes, and therefore affords greater flexibility interms of choosing the dielectric liquid.

Coating aids can be used to improve the uniformity and quality of thecoated or printed electrophoretic ink material. Wetting agents aretypically added to adjust the interfacial tension at thecoating/substrate interface and to adjust the liquid/air surfacetension. Wetting agents include, for example, anionic and cationicsurfactants, and nonionic species, such as silicone orfluoropolymer-based materials. Dispersing agents may be used to modifythe interfacial tension between the capsules and binder, providingcontrol over flocculation and particle settling.

Surface tension modifiers may be added to adjust the air/ink interfacialtension. Polysiloxanes are typically used in such an application toimprove surface leveling while minimizing other defects within thecoating. Surface tension modifiers include, for example, fluorinatedsurfactants, such as, for example, the ZONYL series from DuPont, theFLUORAD series from 3M (St. Paul, Minn.), and the fluoroalkyl seriesfrom Autochem; siloxanes, such as, for example, SILWET from UnionCarbide; and polyethoxy and polypropoxy alcohols. Antifoams, such assilicone and silicone-free polymeric materials, may be added to enhancethe movement of air from within the ink to the surface and to facilitatethe rupture of bubbles at the coating surface. Other useful antifoamsinclude, for example, glyceryl esters, polyhydric alcohols, compoundedantifoams, such as oil solutions of alkylbenzenes, natural fats, fattyacids, and metallic soaps, and silicone antifoaming agents made from thecombination of dimethyl siloxane polymers and silica. Stabilizers suchas UV-absorbers and antioxidants may also be added to improve thelifetime of the ink.

The coacervation approach may utilize an oil/water emulsion. One or morecolloids are coacervated (that is, agglomerated) out of the aqueousphase and deposited as shells around the oily droplets through controlof temperature, pH and/or relative concentrations, thereby creating themicrocapsule. Materials suitable for coacervation include gelatins andgum arabic. See, for example, U.S. Pat. No. 2,800,457, incorporatedherein by reference in its entirety.

In an example complex coacervation process, the display medium to beencapsulated is emulsified with the wall forming material, for example amixture of water, gelatin and gum arabic, at an elevated temperature of,for example, about 30° C. to about 80° C. such as from about 35° C. toabout 75° C. or from about 35° C. to about 65° C. The pH is thenreduced, for example to less than 5 for example from about 4 to about 5such as from about 4.4 to about 4.9, through addition of an acid such asacetic acid and the like, to induce coacervation. The microencapsulatedparticles are then cooled. The material of the wall of the microcapsulesmay then be crosslinked, for example by adding gluteraldehyde and thelike and agitating the mixture in the presence of, for example, urea.

The microcapsules may have a multi-layer wall around the core solidand/or liquid encapsulants. These can be made, for example, by firstforming a thin wall by an interfacial polymerization reaction, andsubsequently forming a second, thicker wall by an in-situ polymerizationreaction or by a coacervation process. The first wall of themicrocapsule may be typically comprised of polyurea, polyurethane,polyamide, polyester, epoxy-amine condensates, silicones and the like.The second wall of the microcapsule may be comprised of condensates ofmelamine-formaldehyde, urea-formaldehyde, resorcinol-formaldehyde,phenol-formaldehyde, gelatin-formaldehyde, or interpolymer complexes oftwo oppositely charged polymers such as gelatin/gum arabic andpoly(styrene sulfonic acid)/gelatin.

A semi-continuous miniemulsion polymerization process may also be usedto encapsulate the electrophoretic display medium, for example asdescribed in U.S. Pat. No. 6,529,313, incorporated herein by referencein its entirety.

A benefit of encapsulating the electrophoretic display medium is thatthe microcapsules can be made to be spherical as shown in FIG. 13 orother than spherical through control of the process. Different shapesmay permit better packing density of the microcapsules and betterdisplay quality.

Once generated, the microcapsules are then located over or adhered toone of the conductive substrates of the device, either directly or viaintermediate layers therebetween. The microcapsules may be adhered tothe conductive side of the substrate, for example the side having aconductive ITO coating thereon. The adhering may be achieved by, forexample, using any suitable binder such as an adhesive or polymer matrixmaterial that is either mixed with the microcapsules prior to coatingthe microcapsules on the substrate, coated onto the substrate beforeplacement of the microcapsules thereon, coated upon the microcapsulesafter placement upon the substrate, or one or more of the above,including all three.

As an adhesive or binder, any material may be used, for exampleincluding polyvinyl alcohol (PVA) or polyurethane such as NEOREZ. Abinder may be used as an adhesive medium that supports and protects thecapsules, as well as binds electrode materials to the capsuledispersion. A binder can be non-conducting, semiconductive, orconductive. Binders are available in many forms and chemical types.Among these are water-soluble polymers, water-borne polymers,oil-soluble polymers, thermoset and thermoplastic polymers, andradiation-cured polymers.

Among water-soluble polymers are various polysaccharides, polyvinylalcohols, N-methylpyrrolidone N-vinylpyrrolidone, various CARBOWAXspecies (Union Carbide), and poly(2-hydroxyethyl acrylate).

The water-dispersed or water-borne systems are generally latexcompositions, for example NEOREZ and NEOCRYL resins (Zeneca Resins),ACRYSOL (Rohm and Haas), BAYHYDROL, (Bayer), and the HP products (CytecIndustries). These are generally lattices of polyurethanes, occasionallycompounded with one or more of acrylics, polyesters, polycarbonates orsilicones, each ending the final cured resin in a specific set ofproperties defined by glass transition temperature, degree of tack,softness, clarity, flexibility, water permeability and solventresistance, elongation modulus and tensile strength, thermoplastic flow,and solids level. Some water-borne systems can be mixed with reactivemonomers and catalyzed to form more complex resins. Some can be furthercross-linked by the use of a cross-linking reagent, such as anaziridine, for example, which reacts with carboxyl groups.

Examples of a water-borne resin and aqueous capsules is provided in U.S.Pat. No. 6,822,782, incorporated herein by reference in its entirety.

Thermoset systems may include the family of epoxies. These binarysystems can vary greatly in viscosity, and the reactivity of the pairdetermines the “pot life” of the mixture. If the pot life is long enoughto allow a coating operation, capsules may be coated in an orderedarrangement in a coating process prior to the resin curing andhardening.

Thermoplastic polymers, which are often polyesters, are molten at hightemperatures. A typical application of this type of product is hot-meltglue. A dispersion of heat-resistant capsules could be coated in such amedium. The solidification process begins during cooling, and the finalhardness, clarity and flexibility are affected by the branching andmolecular weight of the polymer.

Oil or solvent-soluble polymers are often similar in composition to thewater-borne system, with the obvious exception of the water itself. Thelatitude in formulation for solvent systems is enormous, limited only bysolvent choices and polymer solubility. Of considerable concern insolvent-based systems is the viability of the capsule itself; theintegrity of the capsule wall cannot be compromised in any way by thesolvent.

Radiation cure resins are generally found among the solvent-basedsystems. Capsules may be dispersed in such a medium and coated, and theresin may then be cured by a timed exposure to a threshold level ofultraviolet radiation, either long or short wavelength. As in all casesof curing polymer resins, final properties are determined by thebranching and molecular weights of the monomers, oligomers andcross-linkers.

A number of “water-reducible” monomers and oligomers are, however,marketed. In the strictest sense, they are not water soluble, but wateris an acceptable diluent at low concentrations and can be dispersedrelatively easily in the mixture. Under these circumstances, water isused to reduce the viscosity (initially from thousands to hundreds ofthousands centipoise). Water-based capsules, such as those made from aprotein or polysaccharide material, for example, could be dispersed insuch a medium and coated, provided the viscosity could be sufficientlylowered. Curing in such systems is generally by ultraviolet radiation.

The microcapsules may be arranged in abutting, side-by-side relationshipand in embodiments are arranged in a monolayer (that is, themicrocapsules are not stacked) between the conductive substrates.However, more than one layer of microcapsules may also be used.

In a still further embodiment, the display device is comprised of atleast one layer, for example one to ten layers such as one to fourlayers or one to two layers, and specifically one layer, of a binder,for example a transparent binder, containing therein multiple individualcavities or pockets that contain display medium therein. For example, asshown in FIGS. 14 and 15, the binder layer 70 contains multiple cavities72 therein, with cavities filled with fluid 73 and particles 74 of thedisplay medium. If desired, different layers may be used for differentcolor display mediums. The transparent binder layer may be incorporatedinto either rigid or flexible display devices.

This embodiment thus relates to a way of incorporating the displaymedium into a display layer of the device that can easily be applied tocreate large area display devices on a substrate. Essentially, the setsof particles of the display medium are first incorporated into acomposite particle also comprised of a sacrificial binder, that is, abinder that will subsequently be removed. Following incorporation of thecomposite particle into the binder of the binder layer, the sacrificialbinder is removed, and the space occupied in the binder layer by thecomposite particles become cavities or voids containing the particles ofthe display medium. The liquid of the display fluid may then be added tofill the cavities either at the time of removal of the sacrificialbinder or subsequent to removal of the sacrificial binder.

Thus, composite particles comprised of the sets of particles of thedisplay medium and a sacrificial binder are first formed. The compositeparticles may have a size that corresponds substantially to the size ofthe cavities to be formed in the binder layer. For example, thecomposite particles and cavities formed therefrom may have a size ofform about 5 to about 1,000 μm such as from about 10 to about 350 μm orfrom about 20 to about 200 μm.

As the sacrificial binder of the composite particles, use may be made ofwaxes such as polyethylene or polypropylene waxes, for example POLYWAXwaxes from Baker Petrolite. Additional materials that dissolve in thepresence of the fluid of the display medium or that may be melted andremoved from the binder layer may also be used. For example, additionalsacrificial binder materials include a thermoplastic wax, a syntheticmicrocrystalline wax, a crystalline polyethylene wax, or other wax-likematerials that may have a melting point in the range of about 50° C. toabout 200° C. and a sharp melting/crystallization temperature of lessthan about 5° C. Other examples include waxes such as carnauba wax,candelilla wax, castor wax, or the like.

The term wax refers to, for example, a low-melting organic mixture ofcompound of high molecular weight, solid at room temperature, andgenerally similar in composition to fats and oils except that itcontains no glycerides. Some are hydrocarbons, others are esters offatty acids and alcohols. They are classed among the lipids. Waxes arethermoplastic, but because they are not high polymers, they are notconsidered in the family of plastics. Common properties are: waterrepellency, smooth texture, low toxicity, freedom from objectionableodor and color. They are combustible and have good dielectricproperties; soluble in most organic solvents, insoluble in water. Themajor types are as follows: natural: (1) animal (beeswax, lanolin,shellac wax, Chinese insect wax); (2) vegetable (carnauba, candelilla,bayberry, sugar cane); (3) mineral: fossil or earth waxes (ozocerite,ceresin, montan); petroleum waxes (paraffin, micro-crystalline) (slackor scale wax). Synthetic: (1) ethylenic polymers and polyol ether-esters(CARBOWAX, sorbitol); (2) chlorinated naphthalenes (HALOWAX); (3)hydrocarbon type, that is, Fischer-Tropsch synthesis.

Examples of such commercially available materials and their sourcesinclude polyethylene and polypropylene waxes and their modifiedderivatives. One example of a polyethylene wax is POLYWAX 1000,manufactured by the Baker-Petrolite Corporation. This material is anearly crystalline polyethylene wax with a narrow molecular weightdistribution, and, consequently, a narrow melt distribution. Thismaterial retains a low melt viscosity until just above the meltingtemperature, a desirable property for the spherodization of theparticles. Other examples include lower molecular weight POLYWAXmaterials, such as POLYWAX 400, POLYWAX 500, POLYWAX 600, POLYWAX 655,POLYWAX 725, POLYWAX 850, as well as higher molecular weight POLYWAXmaterials such as POLYWAX 2000, and POLYWAX 3000. Other examples ofcommercially available polyethylene waxes include members of the LICOWAXproduct line, available from Clariant. Examples of such materialsinclude: LICOWAX PA520 S, LICOWAX PE130, and LICOWAX PE520, as well asmicronized polyethylene waxes such as CERIDUST 230, CERIDUST 3615,CERIDUST 3620, and CERIDUST 6071.

Examples of commercially available montan waxes include LICOLUB CaW 3,LICOWAX E, LICOWAX OP, all available from Clariant.

A commercially available synthetic form of carnauba wax is PETRONAUBA C,available from Baker-Petrolite Corporation.

Examples of polypropylene waxes include LICOMONT AR504, LICOWAX PP230,CERIDUST 6071, CERIDUST 6072, CERIDUST 6721 (Clariant).

Examples of modified polyethylene waxes include linear alcohol waxessuch as UNILIN alcohols including UNILIN 350, UNILIN 425, UNILIN 550 andUNILIN 700 (Baker-Petrolite Corporation); linear carboxylic acid such asUNICID carboxylic acid polymers including UNICID 350, UNICID 425, UNICID550, and UNICID 700 (Baker-Petrolite Corporation); oxidized polymermaterials such as CARDIS 314, CARDIS 36, CARDIS 320 (Baker-PetroliteCorporation) and oxidized polyethylene waxes such as PETROLITE C-8500,PETROLITE C-7500, PETROLITE E-2020, PETROLITE C-9500, PETROLITE E-1040(Baker-Petrolite Corporation).

Furthermore, in addition to waxes, different polymer materials,including other low polymers, can also be utilized herein so long as thedesired properties and characteristics are produced thereby. Examples ofsuch additional polymers include, for example, maleic anhydride-ethylenecopolymers, maleic anhydride polypropylene copolymers, nylons,polyesters, polystyrene, poly(chloromethylstyrene), and acrylates suchas polymethylmethacrylate.

Commercially available examples of maleic anhydride-ethylene copolymersinclude CERAMER polymers such as CERAMER 1608, CERAMER 1251, CERAMER 6,and CERAMER 5005 (Baker-Petrolite Corporation). Commercially availableexamples of maleic functional polypropylene polymers include X-10036 andX-10016 (Baker-Petrolite Corporation). Commercially available examplesof propylene-ethylene copolymers include PETROLITE copolymers such asPETROLITE EP-700, PETROLITE EP-1104, PETROLITE EP-1100, and PETROLITEEP-1200 (Baker-Petrolite Corporation).

The composite particles may be comprised of from about 25% to about 90%by total weight of the particles of sacrificial binder, for example fromabout 35% to about 80% by total weight or from about 35% to about 70% bytotal weight.

The composite particles are formed by blending the sets of particles ofthe display medium with the sacrificial binder, and forming compositeparticles of the desired size therefrom. Any suitable blending andparticle formation process may be used.

Following formation of the composite particles, an appropriate amount ofthe composite particles, for example from about 10% to about 80% byweight of the binder layer, such as from about 10% to about 70% or fromabout 20% to about 65% by weight of the binder layer, is mixed with thebinder material of the binder layer. A binder layer of desired thicknessmight then formed by any suitable layer forming method.

As the binder of the binder layer, any optically transparent materialmay be used. For example, any of the binders described above for usewith microcapsules may be used. In embodiments, it is desirable for thebinder layer to be able to be plasticized or swollen by the fluid 73 inorder to extract out the sacrificial polymer material to form thecavities. The binder layer should not be decomposed by the fluid 73. Ameans of achieving this is to crosslink the binder layer to enableswelling with solvent without decomposition. The polymeric material usedin embodiments to form the polymeric sheet may include, for example, oneor more polymeric materials selected from elastomeric materials, such asRTV silicone or any of the SYLGARD silicone elastomers from Dow Corning,thermally or UV curable polyurethane resin, thermally or UV curableepoxy resin, and one or more curing agents. Curing may be accomplishedby any suitable method such as thermal, UV, moisture, e-beam, or gammaradiation. Where flexibility is desired, use of silicone elastomers iseffective. However, additional optically transparent binder materialsmay also be used, such as, for example, polyethylene, polyester, epoxy,polyurethane, polystyrene, plexiglass, mixtures thereof and the like.

The binder layer, and thus the display layer of the display device, mayhave a thickness of from about 5 to about 1,000 μm, for example fromabout 10 to about 500 μm or from about 20 to about 350 μm.

In the binder layer, the composite particles act as a template to createthe cavities inside the transparent binder layer. Once formed into alayer or layers, the binder layer or layers are subjected to a treatmentthat removes the sacrificial binder from the composite particlesembedded therein. This may involve, for example, a solvent treatmentprocedure that dissolves the sacrificial binder, a treatment at anelevated temperature to melt and remove the sacrificial binder,combinations thereof, and the like. For example, the sheet may besubjected to an ultrasonic treatment in the presence of the fluid of thedisplay medium. The sacrificial binder diffuses out of the binder layer,leaving the particles of the display medium in the cavities formed bythe composite particles. When the sacrificial binder removal step isconducted using the fluid of the display medium, the sacrificial binderis replaced with the fluid of the display medium, thus leaving thecavities filled with the display medium. The binder layer mayalternatively be swollen with the fluid of the display medium followingthe sacrificial binder removal step, filling the cavities containing theparticles with the display medium fluid.

In embodiments, the display device may also be made to include anabsorptive backplane, for example a light absorptive backplane. Verythin display devices with substantially clear substrates such as ITOcoated glass or ITO coated polymer such as MYLAR may exhibit low opticaldensity, and a washed out appearance with low color saturation. A highlyabsorptive backplane may reduce the light transmission through thedevice, thereby eliminating the washed out appearance of the display.The contrast is greater, and the color saturation appears higher.

The absorptive backplane may desirably have a black color. This may beachieved by any suitable method. For example, a black colored film orpaint may be added onto the back of a transparent substrate. Theabsorptive backplane may be applied either before or after formation ofthe device, for example before formation of a grid pattern on thesubstrate and/or assembly of the film into a display device, or afterassembly of the device but before electrode attachment. Also, thecoloring agent imparting the dark color such as black may beincorporated directly into the conductive substrate layer itself, suchthat the conductive substrate acts as both the conductive layer and theabsorptive backplane.

The display device may also include a color filter. The color filter maybe placed over the display layer, over the top conductive substrate orbetween the top conductive substrate and the display layer(s) having thedisplay medium therein. A color filter is useful when the display deviceotherwise has a two color capability, for example because it iscomprised of a white colored particle set in a colored, for exampleblack, fluid, or because it is comprised of two differently coloredparticles in a display fluid, for example black and white particles. Thecolor filter can impart fuller color capabilities to such displaydevices, for example increasing the two color capability to eight totalcolors as described below.

A multiple color display thus may be achieved by placing filters ofdifferent colors, for example red, green, blue, yellow, cyan or magenta,etc., over the viewing side of individual cells. A color filter of thecolors red, green, and blue can be advantageously used. Moreover, thecolor filter nay comprise stripes of the different colors. The colorfilter is desirably comprised of transparent materials such astransparent polymer films that are tinted with colorant such aspigments, dyes or mixtures of pigments and dyes to have the appropriatecolor yet remain substantially transparent. Thus, the colorant may bepresent in the transparent material of the color filter in an amount offrom about 0.1% to about 10% by weight, for example from about 0.5% toabout 5% by weight.

By placing the color filter over a cell of the display device thatincludes an appropriate number of color switchable reservoirs therein,multiple colors may be achieved. For example, if each color of the colorfilter has a switchable portion of the cell associated therewith so asto be independently driven, multiple colors may be achieved. In otherwords, each colored section of the color filter is associated with anunderlying section of the display layer that may be independentlyaddressed via the conductive substrate so that control of each sectionof the display layer may be made to control the color displayed, asexplained more fully below.

In embodiments, the color filter layer includes a multiplicity of colorfilter sections, each comprised of the different colors of the colorfilter. In this manner, a larger, full color display may be made by thedevice. In these embodiments, the color filter sections may eachcorrespond to a pixel of the display. As such, the color filter layermay include from, for example, about 2 to about 100,000,000, orpotentially more, such as from about 100 to about 50,000,000 or fromabout 1,000 to about 10,000,000, color filter sections.

FIG. 16 illustrates a display device 80 including a display layer 82with individual cells 84 of black and white particles therein. A colorfilter 85 is placed over the cell, the color filter including a red 86,green 87 and blue 88 stripe. In this manner, eight colors may bedisplayed. For example, red may be displayed by driving the cell to havewhite particles 83 display below the red stripe, and black 81 below theblue and green. Green and blue may be similarly displayed by havingwhite particles displayed under these respective stripes of the colorfilter with black under the other two color stripes. Yellow may bederived by having black appear under the blue, and white under both thered and green. Cyan can be derived with white particles displayed underthe green and blue stripes, with black under the red. Magenta may bedisplayed with white under the red and blue stripes of the color filter,and black under the green. White is displayed with white particles underall stripes of the color filter, and black is displayed with black underall of the color filters. Other colors may of course be shown ifdifferent color filter colors are selected.

Display Mediums

Next, various embodiments of the electrophoretic display mediums for usein the electrophoretic display device are described.

In embodiments, the display medium is comprised of at least one fluidand at least one, for example at least two, such as from two to ten orfrom two to four, set(s) of colored particles dispersed in the fluid.

In an embodiment herein, the display medium comprises one or more setsof colored particles dispersed in a fluid system. The fluid may beeither clear/transparent, or it may exhibit a visible color, for examplea different, contrasting color from the color(s) exhibited by the setsof particles dispersed therein. A colored fluid is typically used in adisplay employing a single set of colored particles, for example whiteparticles, with the color of the fluid being a contrasting color otherthan white.

In embodiments, the fluid of the display medium and the set(s) ofparticles therein may have densities that are substantially matched, forexample wherein the densities of these materials are within about 10% ofeach other, or more specifically within 5% of each other or within 2% ofeach other. In other embodiments, the fluid may comprise two immisciblefluids having different densities such that the first immiscible fluidhaving a density less than that of the second immiscible fluid rests ontop of the second immiscible fluid, and each of the sets of particleshas a density in between the densities of the two immiscible fluids suchthat the particles rest at an interface between the two immisciblefluids.

The fluid may comprise from about 10% to about 95% by weight of thedisplay medium, for example from about 30% to about 90% or from about40% to about 80% by weight of the display medium.

The fluid may be comprised of any suitable fluid known in the art foruse in electrophoretic displays. Fluid refers to, for example, amaterial in a liquid state, and is not a gas or air. Of course, air orany other gas may also be present in the reservoirs of the displaydevice, but the fluid of the display medium refers to a fluid in aliquid state. The choice of fluid may be based on concerns of chemicalinertness, density matching to the particles to be suspended thereinand/or chemical compatibility with the particles. In embodiments, thesuspending fluid may have a low dielectric constant (for example, about4 or less, such as about 0.5 to about 2). The viscosity of the fluid maybe relatively low at the temperatures of operation in order to permitthe particles to move therein, for example under the influence of anelectrical field. In embodiments, the fluid may have a kinematicviscosity in the range of about 0.25 centistokes to about 10centistokes, for example from about 0.5 centistokes to about 5centistokes or from about 1 centistoke to about 2 centistokes, at aboutroom temperature (about 23° C. to about 27° C.). The fluid may bedielectric and substantially free of ions. The fluid also may haveminimum solvent action on the colored particles therein, and a specificgravity substantially equal to the colored particles, for example withinabout 10% of each other. Additionally, the fluid may be chosen to be apoor solvent for some polymers, which is advantageous for use in thefabrication of particles because it increases the range of polymericmaterials useful in fabricating particles.

The fluid may include therein a thermally reversible gelling agenthaving a melting point temperature of at least about 35° C., for exampleas described in co-pending application Ser. No. 11/169,924, incorporatedherein by reference in its entirety.

Organic solvents such as halogenated organic solvents, saturated linearor branched hydrocarbons, silicone oils, and low molecular weighthalogen-containing polymers are a few suitable types of fluids that maybe used. Organic solvents may include, for example, epoxides such as,for example, decane epoxide and dodecane epoxide, vinyl ethers such as,for example, cyclohexyl vinyl ether, and aromatic hydrocarbons such as,for example, toluene and naphthalene. Halogenated organic solvents mayinclude, for example, tetrafluorodibromoethylene, tetrachloroethylene,trifluorochloroethylene, 1,2,4-trichlorobenzene, carbon tetrachloride,mixtures thereof and the like. These materials may have high densities.Hydrocarbons may include, for example, decane, dodecane, tetradecane,xylene, toluene, hexane, cyclohexane, benzene, the aliphatichydrocarbons in the ISOPAR™ (Exxon), NORPAR™ (a series of normalparaffinic liquids from Exxon), SHELL-SOL™ (Shell), and SOL-TROL™(Shell) series, naphtha, and other petroleum solvents. These materialsmay have low densities. Examples of silicone oils include octamethylcyclosiloxane and higher molecular weight cyclic siloxanes, poly(methylphenyl siloxane), hexamethyldisiloxane and polydimethylsiloxane. Thesematerials may have low densities. Low molecular weighthalogen-containing polymers may include, for example,poly(chlorotrifluoroethylene) polymer or KRYTOX™ polymers (Dupont).

Typically, hydrocarbon fluids such as ISOPAR M are used forelectrophoretic ink applications due to their low cost, good dielectricstrength, low volatility, and nonreactivity.

In embodiments, the aliphatic hydrocarbons may cause degradation ofperformance, for example when non-crosslinked emulsion aggregationparticles are used as the colored particles of the display medium and/orwhen the colored particles are imparted with a charge by treatment witha surface coating that can be desorbed from the particle surface in thepresence of an aliphatic hydrocarbon. Thus, it may be desirable to useas the fluid of the display medium a nonswelling fluid such as asilicone fluid. A commercially available silicone fluid includes DOW200, a polydimethylsiloxane polymer available from Dow Corning. Otherexamples of suitable silicone fluids include polydimethylsiloxane fluidsavailable from Gelest Corporation such as trimethylsiloxy terminatedfluids DMS-T00, DMS-T01, DMS-T01.5, DMS-T02, DMS-T03, DMS-T05, DMS-T07,DMS-T11; cyclomethicones such as SIO6700.0, SID2650.0, SID4625.0 (alsoknown as D4, D5, and D6 fluids, respectively); phenylmethylsiloxanessuch as PMM-0011, PDM-7040; fluorosilicones such as SIB1816.0;polydiethylsiloxanes such as DES-T03, DES-T11; branched and lowviscosity phenyltris(trimethylsiloxy)silane fluids such as SIP6827.0,phenethyltris(trimethylsiloxy)silane fluids such as SIP6722.8, and thelike.

If colored, the fluid may be colored by any suitable means in the art,including through the inclusion of suitable dispersible colorants suchas dyes and/or dispersible pigments therein.

In embodiments, the fluid is substantially free of charge controladditives and other ionic species that may affect the charging behaviorof the display medium and/or the particles dispersed therein. However,in other embodiments, the fluid may contain additives such as surfacemodifiers to modify the surface energy or charge of the particles andsuch as charge control agents, dispersants, and/or surfactants.

The display medium may be comprised of two immiscible liquids. Such atwo-layer fluid system may be achieved using two fluids with differingdensities and that are immiscible with each other. For example, 3M'sfluoroether and Exxon's ISOPAR™ are a suitable combination of immisciblefluids. Fluoroether, being denser, rests on the bottom, while ISOPAR™,being less dense, rests on top. The particles of the display medium mayhave a density that is in between the densities of the two immiscibleliquids so that they rest at the interface between the two layers.

Advantages of using two immiscible liquids may include that the restposition of the particles is at the interface of the two immiscibleliquids (which may be near the middle portion of the reservoir) ratherthan at the bottom of the reservoir in which the display liquid iscontained. This may avoid potential adhesion between the particles andthe reservoir bottom. In addition, the switching time may be made fasterbecause the particles only need to travel a portion of the distance ofthe reservoir in switching positions to display a different color to aviewer, and the particles rested at the interface may break loose moreeasily compared to particles resting at the bottom, which may increaseparticle stability and product life.

Various embodiments of particle sets to be dispersed in the fluid of thedisplay medium are next described.

In embodiments, the display medium includes at least one set ofparticles exhibiting substantially the same color. The display mediummay be comprised of one set of colored particles, including at leasttwo, such as from two to ten or from two to four, sets of differentlycolored particles dispersed in the fluid. Color refers to, for example,the overall absorption characteristic within the range of wavelengths ofthe electromagnetic spectrum. Substantially the same color herein refersto, for example, particles exhibiting substantially the same hue andcontrast (darkness/lightness) as other particles in the set. Coloredparticles of different sets of particles in the display medium exhibit acolor, that is, an absorption characteristic, different from each other.For example, if a first set of particles exhibits a yellow color, then asecond differently colored set of particles will exhibit a differentshade (hue and/or contrast) of yellow or a different color altogether,for example such as cyan or magenta.

A display medium may include two sets of differently colored particles,for example black particles and white particles. In embodiments, thedisplay medium comprises at least three differently colored sets ofparticles. As examples, the three sets of colored particles may comprisethe three subtractive primary colors yellow, cyan and magenta, or maycomprise red, blue and green. An example display medium containing foursets of differently colored particles may comprise yellow, cyan, magentaand black. Additional differently colored sets of particles, for examplefor highlight coloring, may be included as additional sets of coloredparticles in any embodiment described herein.

Each set of same colored particles in the display medium may comprisefrom about 5% to about 50% by weight, for example from about 5% to about40% or from about 5% to about 30% by weight, of the display medium.

In embodiments, described is a low electrical conductivityelectrophoretic display medium, for example having a conductivity on theorder of about 10⁻¹¹ to about 10⁻¹⁵ S/m, such as from about 10⁻¹² toabout 10⁻¹⁴ S/m or from about 10⁻¹² to about 10⁻¹³ S/m. The conductivityof the display medium is thus comparable to that of the dielectricfluid. The particles of the display medium may become charged by theapplication of a high electric field thereto, which may also be referredto as field-induced or in situ charging, in which particle charging isdependent on, for example, the field strength and the charging time (ornumber of charging cycles). Following charging, the particles may have acharge (charge to mass ratio) on the order of microcoulombs (μC) pergram (that is, on the order of 10⁻⁶ C/g), such as from about ±0.1 toabout ±20 μC/g, from about ±0.2 to about ±10 μC/g or from about ±0.3 toabout ±5 μC/g.

In prior display mediums, the particles were typically charged by addinga charge control agent, which is capable of ionic dissociation, to thefluid during preparation of the non-aqueous ink dispersion. Dissociationof the charge control agent into positive and negative ionic species inthe dielectric fluid results in preferential surface absorption of ionsof one polarity by the particles, and the particles therefore becomecharged. The resulting dispersion contains a complex mixture ofparticles including charged particles, excess free ions andcounter-ions. Due to the presence of excess free ions, theelectrophoretic ink is also characterized by high electricalconductivity, which increases with concentration of the added chargecontrol agent and is typically 100-1000 times higher compared with thedielectric fluid. High conductivity of the ink results in increasedpower consumption and may result in slower switching speed of thedisplay. Moreover, the presence of excess free ions in the displaymedium makes it possible for many of the particles to switch to a wrongsign/polarity during collisions between particles in use, which maydegrade image quality and response time.

The display medium, including the fluid and particle sets therein, ofembodiments herein may thus be made to be substantially free of chargecontrol additives and similar excess ionic species affecting thecharging characteristics and/or conductivity of the display medium.Substantially free of ions herein refers, for example, to the displaymedium being free of ionic species to the extent that the aforementionedconductivity values may be achieved. As a result, the display mediumherein is able to exhibit the aforementioned low conductivityproperties.

As a result of the desired absence of charge control additives in thedisplay medium, the particles of the sets of particles of the displaymedium need to be made to include a capability of exhibiting the lowcharging property by other methods. Such may be accomplished, forexample, by the formation of the particles in the presence of asurfactant and/or water, wherein small amounts of these materials may beincorporated into the particles during formation. Other components thatcould impart the charge to the particles include polymerizationinitiators such as APS (ammonium persulfate), chain transfer agents suchas DDT (dodecylthiol), or acidic/basic functional groups in the polymerbackbone that may be exposed or partially exposed on the particlesurface. These materials may act as charge species in the particles,imparting an almost negligible charge at time zero but that whichenables the particles to be charged, for example through application ofa high electric field as will be described more fully below, to the lowcharge values described above. These materials are part of the particlesand substantially do not become dissociated in the display medium,thereby enabling the display medium to maintain the low conductivity.Moreover, unlike prior systems requiring the presence of ionic speciesin the medium that permit the display to degrade in performance overtime, for example through the generation of wrong sign particles and/orloss of sufficient ionic species in the medium, the particles herein donot generate ionic species and do not require the presence of ionicspecies for charging, and thus are not subject to such degradationrisks.

As the particles of the display medium, any particle made by anysuitable process may be used, so long as the particles are capable ofexhibiting the low charge property discussed above. Thus, particles madeby both physical grinding methods, in which the material of theparticles is formed as a mass that is then crushed and ground to thedesired average particle size, and chemical build-up methods, in whichthe particles are grown individually within a reaction medium to thedesired average particle size, both of which types of methods are wellknown in the toner art, may be used. The particles nay be made to havean average size of from, for example, about 5 nm to about 100 μm, suchas from about 10 nm to about 50 μm or from about 0.5 μm to about 25 μm.The particles typically have a size less than the size of the reservoirsof the display device in which the display medium will be contained sothat the particles are free to move within the reservoirs.

The particles may be neat pigments, dyed (laked) pigments,pigment/polymer composites, dyed or pigmented agglomerated polymerparticles and the like. As the colorant of the particles, dyes, pigment,mixtures of dyes, mixtures of pigments or mixtures of dyes and pigmentsmay be used. Particles and/or colorant of particles may also includelaked, or dyed, pigments, in which a dye is precipitated on theparticles or the particles are stained with a dye such as metal salts ofreadily soluble anionic dyes, for example dyes of azo, triphenylmethaneor anthraquinone structure containing one or more sulphonic orcarboxylic acid groupings precipitated by a calcium, barium or aluminumsalt.

Typical manufacturing techniques for the above particles are drawn fromthe liquid toner and other arts and include ball milling, attrition, jetmilling, and the like. A pigmented polymer particle may be made by, forexample, compounding a pigment in the polymer. The composite material isthen (wet or dry) ground to a desired size. It may then optionally beadded to a carrier liquid and milled under high shear for several hoursto a final particle size and/or size distribution.

Chemical processes that may be used in forming the particles include,for example, emulsion aggregation, dispersion polymerization, mini- ormicro-emulsion polymerization, suspension polymerization, precipitation,phase separation, solvent evaporation, in situ polymerization, or anyprocess of microencapsulation.

Polymers that may be used for the pigmented particles include, forexample, polystyrene, polyethylene, polypropylene, phenolic resins,ethylene-vinyl acetate copolymers, polyesters, polyacrylates,polymethacrylates, ethylene acrylic acid or methacrylic acid copolymers,acrylic copolymers and terpolymers and the like. Specific exampleinclude, for example, polyethylene, polypropylene,polymethylmethacrylate, polyisobutylmethacrylate, polystyrene,polybutadiene, polyisoprene, polyisobutylene, polylauryl methacrylate,polystearyl methacrylate, polyisobornyl, methacrylate, poly-t-butylmethacrylate, polyethyl methacrylate, polymethyl acrylate, polyethylacrylate, polyacrylonitrile, and copolymers of two or more of thesematerials.

While pigment/polymer composite particles, for example compositeparticles created by a physical-chemical process such asgrinding/attrition of pigment/polymer or by surface treatment/graftingof stabilizing polymeric groups on the surface, may be used herein, suchcomposite particles may have polydisperse particles that exhibitvariable charging characteristics. Thus, in embodiments, the particlesfor the display medium are emulsion aggregation particles, for exampleincluding polyester resin based emulsion aggregation particles andstyrene-acrylate or acrylate resin based emulsion aggregation particles.Such particles are chemically grown and tend to be substantiallymonodisperse in size and substantially spherical in shape. Anotheradvantage to emulsion aggregation particles is that the particle surfaceis substantially completely passivated by the binder resin, which mayeliminate the contribution of the colorant, such as pigment, to theparticle charge.

Examples of suitable polyester resins for the emulsion aggregationparticles include polyethylene terephthalate, polypropyleneterephthalate, polybutylene terephthalate, polypentylene terephthalate,polyhexylene terephthalate, polyheptadene terephthalate, polyoctaleneterephthalate, polyethylene sebacate, polypropylene sebacate,polybutylene sebacate, polyethylene adipate, polypropylene adipate,polybutylene adipate, polypentylene adipate, polyhexalene adipate,polyheptadene adipate, polyoctalene adipate, polyethylene glutarate,polypropylene glutarate, polybutylene glutarate, polypentyleneglutarate, polyhexylene glutarate, polyheptadene glutarate, polyoctaleneglutarate polyethylene pimelate, polypropylene pimelate, polybutylenepimelate, polypentylene pimelate, polyhexylene pimelate, polyheptadenepimelate, poly(propoxylated bisphenol fumarate), poly(propoxylatedbisphenol succinate), poly(propoxylated bisphenol adipate),poly(propoxylated bisphenol glutarate), mixtures, copolymers orcombinations thereof, and the like.

Polyester toner particles, formed by the emulsion aggregation process,are illustrated in a number of patents, such as U.S. Pat. No. 5,593,807,U.S. Pat. No. 5,290,654. U.S. Pat. No. 5,308,734, and U.S. Pat. No.5,370,963, each of which is incorporated herein by reference in theirentirety. Further examples of suitable polyester particles include thosehaving lithium and/or sodium sulfonated polyester resin as disclosed ina number of patents, such as U.S. Pat. Nos. 6,387,581 and 6,395,445,each of which is incorporated herein by reference in their entirety. Thepolyester may comprise any of the polyester materials described in theaforementioned references.

An example process for preparing the polyester based emulsionaggregation particles may comprise charging a polyester resin emulsion,for example an aqueous based emulsion optionally containing one or moresurfactants, into a reactor, and adding a colorant to the reactor whilestirring. A wax dispersion may optionally be added. The mixture isstirred and heated to a desired temperature, for example from about 40°C. to about 70° C., such as from about 45° C. to about 70° C. or fromabout 40° C. to about 65° C. A solution of an aggregating agent ispumped into the mixture to initiate growth/aggregation of the polyesterparticles. An additional amount of resin emulsion may then be added,where it is desired to form a shell that is substantially free ofcoloring agent such as dyes, pigments or mixtures thereof on the coreaggregated colored particles. The temperature of the reactor may then beraised towards the end of the reaction to, for example, from about 45°C. to about 75° C., such as from about 50° C. to about 75° C. or fromabout 45° C. to about 70° C., to allow for appropriate spherodizationand coalescence to achieve the desired average particle size and shape.The slurry may be cooled, washed and dried.

Examples of suitable acrylate resin binders for the emulsion aggregationparticles include, for example, polymers such as poly(styrene-alkylacrylate), poly(styrene-1,3-diene), poly(styrene-alkyl methacrylate),poly(styrene-alkyl acrylate-acrylic acid),poly(styrene-1,3-diene-acrylic acid), poly(styrene-alkylmethacrylate-acrylic acid), poly(alkyl methacrylate-alkyl acrylate),poly(alkyl methacrylate-aryl acrylate), poly(aryl methacrylate-alkylacrylate), poly(alkyl methacrylate-acrylic acid), poly(styrene-alkylacrylate-acrylonitrile-acrylic acid),poly(styrene-1,3-diene-acrylonitrile-acrylic acid), and poly(alkylacrylate-acrylonitrile-acrylic acid); the latex contains a resinselected from the group consisting of poly(styrene-butadiene),poly(methylstyrene-butadiene), poly(methyl methacrylate-butadiene),poly(ethyl methacrylate-butadiene), poly(propyl methacrylate-butadiene),poly(butyl methacylate-butadiene), poly(methyl acrylate-butadiene),poly(ethyl acrylate-butadiene), poly(propyl acrylate-butadiene),poly(butyl acrylate-butadiene), poly(styrene-isoprene),poly(methylstyrene-isoprene), poly(methyl methacrylate-isoprene),poly(ethyl methacrylate-isoprene), poly(propyl methacrylate-isoprene),poly(butyl methacrylate-isoprene), poly(methyl acrylate-isoprene),poly(ethyl acrylate-isoprene), poly(propyl acrylate-isoprene),poly(butyl acrylate-isoprene); poly(styrene-propyl acrylate),poly(styrene-butyl acrylate), poly(styrene-butadiene-acrylic acid),poly(styrene-butadiene-methacrylic acid),poly(styrene-butadiene-acrylonitrile-acrylic acid), poly(styrene-butylacrylate-acrylic acid), poly(styrene-butyl acrylate-methacrylic acid),poly(styrene-butyl acrylate-acrylonitrile), and poly(styrene-butylacrylate-acrylonitrile-acrylic acid).

Acrylate toner particles created by the emulsion aggregation process areillustrated in a number of patents, such as U.S. Pat. No. 5,278,020,U.S. Pat. No. 5,346,797, U.S. Pat. No. 5,344,738, U.S. Pat. No.5,403,693, U.S. Pat. No. 5,418,108, and U.S. Pat. No. 5,364,729, each ofwhich is incorporated herein by reference in their entirety. Theacrylate may comprise any of the materials described in theaforementioned references. In embodiments, the acrylate polymer may be astyrene-acrylate copolymer, such as styrene-butyl acrylate that may alsobe comprised of β-carboxyethylacrylate.

Thus, the binder may be specifically comprised of a styrene-alkylacrylate, for example a styrene-butyl acrylate copolymer resin, or astyrene-butyl acylate-β-carboxyethyl acrylate polymer resin.

The monomers used in making the acrylate polymer binder may include anyone or more of, for example, styrene, acrylates such as methacrylates,butylacrylates, β-carboxyethyl acrylate (β-CEA), etc., butadiene,isoprene, acrylic acid, methacrylic acid, itaconic acid, acrylonitrile,benzenes such as divinylbenzene, etc., and the like. Known chaintransfer agents can be utilized to control the molecular weightproperties of the polymer. Examples of chain transfer agents includedodecanethiol, dodecylmercaptan, octanethiol, carbon tetrabromide,carbon tetrachloride, and the like in various suitable amounts, forexample of about 0.1 to about 10 percent by weight of monomer, andpreferably of about 0.2 to about 5 percent by weight of monomer. Also,crosslinking agents such as decanedioldiacrylate or divinyl benzene maybe included in the monomer system in order to obtain higher molecularweight polymers, for example in an effective amount of about 0.01percent by weight to about 25 percent by weight, preferably of about 0.5to about 10 percent by weight.

An example method for making acrylate based emulsion aggregationparticles may include first mixing resin emulsion, for example anaqueous based emulsion optionally containing one or more surfactants, acolorant, and a coagulating agent at a temperature at or above the glasstransition temperature (Tg) of the resin, such as 5° C. to about 50° C.above the Tg of the resin, which Tg is usually in the range of fromabout 50° C. to about 80° C. or is in the range of from about 52° C. toabout 65° C. The particles are permitted to grow or aggregate to adesired size. An outer shell material for the aggregated particles, forexample consisting essentially of binder resin that is substantiallyfree of coloring agent such as dyes, pigments or mixtures thereof on thecore aggregated colored particles, may then be added, for example toform a shell on the aggregated particles having a thickness of about 0.1to about 2 micron. The aggregation is then halted, for example with theaddition of a base. The particles may then be coalesced, for example atan elevated temperature such as from about 60° C. to about 98° C., untila suitable shape and morphology is obtained. Particles are thenoptionally subjected to further processing, for example wet sieved,washed by filtration, and/or dried.

As surfactants for use in making emulsion aggregation particles asdiscussed above, examples include anionic, cationic, nonionicsurfactants and the like.

Anionic surfactants include sodium dodecylsulfate (SDS), sodium dodecylbenzene sulfonate, sodium dodecylnaphthalene sulfate, dialkylbenzenealkyl, sulfates and sulfonates, abitic acid, and the NEOGEN brandof anionic surfactants. NEOGEN R-K available from Daiichi Kogyo SeiyakuCo. Ltd. (Japan), or Tayca Power BN2060 from Tayca Corporation (Japan)consist primarily of branched sodium dodecyl benzene sulphonate.

Examples of cationic surfactants include dialkyl benzene alkyl ammoniumchloride, lauryl trimethyl ammonium chloride, alkylbenzyl methylammonium chloride, alkyl benzyl dimethyl ammonium bromide, benzalkoniumchloride, cetyl pyridinium bromide, C₁₂, C₁₅, C₁₇ trimethyl ammoniumbromides, halide salts of quaternized polyoxyethylalkylamines, dodecylbenzyl triethyl ammonium chloride, MIRAPOL and ALKAQUAT available fromAlkaril Chemical Company, SANISOL (benzalkonium chloride), availablefrom Kao Chemicals, and the like. SANISOL, B-50 consists primarily ofbenzyl dimethyl alkonium chloride.

Examples of nonionic surfactants include polyvinyl alcohol, polyacrylicacid, methalose, methyl cellulose, ethyl cellulose, propyl cellulose,hydroxy ethyl cellulose, carboxy methyl cellulose, polyoxyethylene cetylether, polyoxyethylene lauryl ether, polyoxyethylene octyl etherpolyoxyethylene octylphenyl ether, polyoxyethylene oleyl ether,polyoxyethylene sorbitan monolaurate, polyoxyethylene stearyl ether,polyoxyethylene nonylphenyl ether, dialkylphenoxy poly(ethyleneoxy)ethanol, available from Rhone-Poulene Inc. as IGEPAL CA-210, IGEPALCA-520, IGEPAL CA-720, IGEPAL, CO-890, IGEPAL CO-720, IGEPAL CO-290,IGEPAL CA-210, ANTAROX 890 and ANTAROX 897. ANTAROX 897 consistsprimarily of alkyl phenol ethoxylate.

The toner preparation is typically carried out in an aqueous (water)environment as detailed above, and the electrophoretic ink is annon-aqueous environment (oil). When the toner is prepared, it is given afinal water wash to remove excess surfactant. Trace amounts of residualsurfactant on the surface of the toner particle, or trapped within theparticle itself, may remain and contribute to the low conductivity ofthe particles. However, the amount of surfactant that actually gets intothe oil is very low, since it prefers to be in water. As a result, thefluid medium has a desired low conductivity.

In embodiments, the emulsion aggregation particles are made to have anaverage particle size of from about 0.5 to about 25 μm, for exampleabout 5 to about 15 μm or about 5 to about 12 μm. The particle size maybe determined using any suitable device, for example a conventionalCoulter counter.

The emulsion aggregation particles also may have a substantiallymonodisperse size such that the upper geometric standard deviation (GSD)by volume for (D84/D50) is in the range of from about 1.1 to about 1.25.The particle diameters at which a cumulative percentage of 50% of thetotal toner particles are attained are defined as volume D50, and theparticle diameters at which a cumulative percentage of 84% are attainedare defined as volume D84. These aforementioned volume average particlesize distribution indexes GSDv can be expressed by using D50 and D84 incumulative distribution, wherein the volume average particle sizedistribution index GSDv is expressed as (volume D84/volume D50). Theupper GSDv value for the toner particles indicates that the tonerparticles are made to have a very narrow particle size distribution.

The emulsion aggregation particles also may be made to be highlycircular, thereby exhibiting better flow properties with respect tomovement within the display medium. In other words, rounder/smootherparticles have a higher electrophoretic mobility, and thus a fasterresponse time within the display. The circularity is a measure of theparticles closeness to a perfect sphere. A circularity of 1 identifies aparticle having the shape of a perfect circular sphere. The emulsionaggregation particles may have an average circularity of about 0.92 toabout 0.99, for example from about 0.94 to about 0.98 or from about 0.95to about 0.97. The circularity may be determined using the known MalvernSysmex Flow Particle Image Analyzer FPIA-2100.

In embodiments, the binder of the particles is comprised of a mixture oftwo binder materials of differing molecular weights, such that thebinder has a bimodal molecular weight distribution (that is, withmolecular weight peaks at least at two different molecular weightregions). For example, the binder may be comprised of a first lowermolecular weight binder, for example a non-crosslinked binder, and asecond high molecular weight binder, for example a crosslinked binder.The first binder may have a number average molecular weight (Mn), asmeasured by gel permeation chromatography (GPC), of from, for example,about 1,000 to about 30,000, and more specifically from about 5,000 toabout 15,000, a weight average molecular weight (Mw) of from, forexample, about 1,000 to about 75,000, and more specifically from about25,000 to about 40,000, and a glass transition temperature of from, forexample, about 40° C. to about 75° C. The second binder may have asubstantially greater number average and weight average molecularweight, for example over 1,000,000 for Mw and Mn, and a glass transitiontemperature of from, for example, about 35° C. to about 75° C. The glasstransition temperature may be controlled, for example, by adjusting theamount of acrylate in the binder. For example, a higher acrylate contentcan reduce the glass transition temperature of the binder. The secondbinder may be referred to as a gel, which is a highly crosslinkedpolymer, due to the extensive gelation and high molecular weight of thelatex. In this embodiment, the gel binder may be present in an amount offrom about 0% to about 50% by weight of the total binder, preferablyfrom about 8% to about 35% by weight of the total binder.

The first, lower molecular weight binder may be selected from among anyof the aforementioned polymer binder materials. The second gel bindermay be the same as or different from the first binder. For example, foracrylate binders, the second gel binder may be comprised of highlycrosslinked materials such as poly(styrene-alkyl acrylate),poly(styrene-butadiene), poly(styrene-isoprene), poly(styrene-alkylmethacrylate), poly(styrene-alkyl acrylate-acrylic acid),poly(styrene-alkyl methacrylate-acrylic acid), poly(alkylmethacrylate-alkyl acrylate), poly(alkyl methacrylate-aryl acrylate),poly(aryl methacrylate-alkyl acrylate), poly(alkyl methacrylate-acrylicacid), poly(styrene-alkyl acrylate-acrylonitrileacrylic acid), andpoly(alkyl acrylate-acrylonitrile-acrylic acid), and/or mixturesthereof. In embodiments, the gel binder is the same as the first binder,and both are a styrene acrylate, for example a styrene-butyl acrylate orstyrene-butyl acrylate of styrene-butyl acrylate-β-carboxy ethylacrylate. The higher molecular weight of the second gel binder may beachieved by, for example, including greater amounts of styrene in themonomer system, including greater amounts of crosslinking agent in themonomer system and/or including lesser amounts of chain transfer agents.

In still further embodiments, the emulsion aggregation particles have acore-shell structure. In this embodiment, the core is comprised of theparticle materials discussed above, including at least the binder andthe colorant. Once the core particle is formed and aggregated to adesired size, a thin outer shell is then formed upon the core particle.The shell may be comprised of only binder material, although othercomponents may be included therein if desired. The shell may becomprised of a latex resin that is the same as a latex of the coreparticle. The shell latex may be added to the core aggregates in anamount of about 5 to about 40 percent by weight of the total bindermaterials, for example in an amount of about 5 to about 30 percent byweight of the total binder materials. The shell or coating on theaggregates may have a thickness wherein the thickness of the shell isabout 0.2 to about 1.5 μm, for example about 0.3 to about 1.2 μm or fromabout 0.5 to about 1 μm.

The total amount of binder, including core and shell if present, may bein the range of from about 60 to about 95% by weight of the emulsionaggregation particles (toner particles exclusive of external additives)on a solids basis, for example from about 70 to about 90% by weight ofthe particles.

The particles may also be made by emulsion aggregation starting fromseed particles derived via a stable free-radical polymerization method.Such stable free-radical polymerization (SFRP) processes are known inthe art, for example as described in U.S. Pat. No. 5,322,912, the entiredisclosure of which is totally incorporated herein by reference. In theSFRP processes, propagating chains of the polymer are referred to as“pseudo-living” because the stable free-radical agent adds to apropagating chain and the chain is temporarily, but reversibly,terminated. This allows for the formation of block copolymers that canincorporate monomers that will enhance the particle charge. The monomersdue to this block character can be at the particle surface (especiallyif they are formed from hydrophilic monomers) and thus the charge of theparticle will be enhanced. Such monomers can be amines such asaminoethylacrylate or methacrylate, sulfonates such asstyrenesulfonates, acids such as β-carboxyethylacrylate or methacrylate,or any heteroatom monomers that can be ionized or quaternized. Theresultant polymers of SFRP are dispersed in an aqueous phase to form thestarting latex of the emulsion aggregation processes discussed above.Thus, SFRP may be used to form any of the polymers described above asbinders for the emulsion aggregation particles.

In addition to the polymer binder and the colorant, the particles mayalso contain a wax dispersion. Linear polyethylene waxes such as thePOLYWAX®, line of waxes available from Baker Petrolite are useful. Ofcourse, the wax dispersion may also comprise polypropylene waxes, otherwaxes known in the art, including carnauba wax and the like, andmixtures of waxes. The toners may contain from, for example, about 1 toabout 15% by weight of the particles, on a solids basis, of the wax, forexample from about 3 to about 12% or from about 5 to about 10% byweight.

In addition, the colored particles may also optionally contain acoagulant and/or a flow agent such as colloidal silica. Suitableoptional coagulants include any coagulant known or used in the art,including the well known coagulants polyaluminum chloride (PAC) and/orpolyaluminum sulfosilicate (PASS). The coagulant is present in the tonerparticles, exclusive of external additives and on a dry weight basis, inamounts of from 0 to about 3% by weight of the toner particles, forexample from about greater than 0 to about 2% by weight of the tonerparticles. The flow agent, if present, may be any colloidal silica suchas SNOWTEX OL/OS colloidal silica. The colloidal silica is present inthe toner particles, exclusive of external additives and on a dry weightbasis, in amounts of from 0 to about 15% by weight of the tonerparticles, for example from about greater than 0 to about 10% by weightof the toner particles.

Although not required, the toner may also include additional knownpositive or negative charge additives in effective suitable amounts of,for example, from about 0.1 to about 5 weight percent of the toner, suchas quaternary ammonium compounds inclusive of alkyl pyridinium halides,bisulfates, organic sulfate and sulfonate compositions such as disclosedin U.S. Pat. No. 4,338,390, cetyl pyridinium tetrafluoroborates,distearyl dimethyl ammonium methyl sulfate, aluminum salts or complexes,and the like.

In embodiments, one or more sets of the colored particles incorporatedinto the display medium comprise crosslinked emulsion aggregationparticles. The crosslinking may be achieved by any suitable method,including, for example, thermal curing or radiation, for example UV,curing. Crosslinked refers to, for example, the high molecular weightstate achieved by including crosslinkable monomer or oligomer additivesin a composition along with an initiator and exposing the composition toa curing environment (for example, elevated temperature for thermalcuring or UV light for radiation curing) to effect curing of theadditives. Other components of the composition, for example the otherbinder resin components, may also participate in the crosslinking.

Gel content may be used to define the extent of crosslinking in theparticles. The crosslinking forms a gel portion that has significantlyincreased strength and less solvent solubility with respect to theindividual polymer chains. Gel content refers to the proportion of thepolymer chains of the polymer particles that have been crosslinked,thereby constituting a part of the gel network. In embodiments, theparticles may have a gel content from about 10 percent to about 100percent, for example from about 20 to about 80 percent or from about 25to about 75 percent.

The gel content of the polymer particles is quantitatively measured, forexample by continuously extracting, for example by soxhlet extraction,the reaction product after crosslinking processing is complete, by whichthe weight of the crosslinked polymer material can be obtained. Acontinuous extraction method allows polymers that are soluble to beremoved from the mass of crosslinked polymer that typically is notsoluble in most or any solvents. Accordingly, the use of a solvent inwhich the polymer is soluble, and in which the crosslinked portions areinsoluble, is used for the procedure. By dividing the weight of thecrosslinked polymer material by the total weight of the material thatwas continuously extracted, and multiplying by 100, the gel contentvalue may be obtained. The degree of crosslinking may be regulated bycontrolling the time and/or intensity of the crosslinking procedure,and/or by the concentration of the crosslinkable materials in theparticles.

As was discussed above, hydrocarbon fluids such as ISOPAR M are adesirable fluid to use for an electrophoretic display medium. However,using such a fluid system with emulsion aggregation particle sets mayresult in device degradation, for example as a result of the fluidcausing swelling of the emulsion aggregation resin and leaching out ofthe component materials such as wax, surface treatment reagents, etc.,from the swollen particles.

Crosslinkable particles may be prepared by including in the binder oneor more crosslinking additives. After the emulsion aggregation particleformation process described above, the toner particles are subjected toa radiation curing step, for example comprising UV radiation, to effectthe crosslinking process, resulting in a robust particle with excellentresistance to solvent swelling, and also having enhanced resistance tosoftening/melting at elevated temperatures.

The crosslinking additives may be added to any type of emulsionaggregation resin binder to permit the particles made therefrom to be UVcrosslinkable. The one or more crosslinking additives thus may beincluded in either acrylate or polyester type emulsion aggregationresins. The additive may be present in an amount of from, for example,about 0.5 to about 50% by weight, for example from about 0.5 to about25% by weight or from about 1 to about 20% by weight of the total binderin the particles.

Examples of the crosslinking additives include multifunctional acrylatessuch as diacrylates, triacrylates, tetraacrylates, and the like. Forexample, the multifunctional acrylate monomer or oligomer, may includediacrylates such as propoxylated neopentyl glycol diacrylate (availablefrom Atofina as Sartomer SR 9003), 1,6-hexanediol diacrylate (SartomerSR 238), tripropylene glycol diacrylate, dipropylene glycol diacrylate,aliphatic diacrylate oligomer (CN 132 from Atofina), aliphatic urethanediacrylate (CN 981 from Atofina), aromatic urethane diacrylate (CN 976from Atofina) and the like, triacrylate or higher functionality monomersor oligomers such as amine modified polyether acrylates (available as PO83 F, LR 8869, and/or LR 8889 from BASF Corporation), trimethylolpropane triacrylate (Sartomer SR 351), tris (2-hydroxy ethyl)isocyanurate triacrylate (Sartomer SR 368), aromatic urethanetriacrylate (CN 970 from Atofina), dipentaerythritolpenta-/hexa-acrylate, pentaerythritol tetraacrylate (Sartomer SR 295),ethoxylated pentaerythritol tetraacrylate (Sartomer SR 494),dipentaerythritol pentaacrylate (Sartomer SR 399) and the like, ormixtures of any of the foregoing. Additional examples of suitablecrosslinking additives include chlorinated polyester acrylate (SartomerCN 2100), amine modified epoxy acrylate (Sartomer CN 2100), aromaticurethane acrylate (Sartomer CN 2901), and polyurethane acrylate (LaromerLR 8949 from BASF). Other unsaturated curable resins that may be usedare described in U.S. Patent Publication No. 2005/0137278 A1, which isherein incorporated by reference in its entirety.

A crosslinking initiator is also included in the crosslinking additives.Photoinitiators such as 2,4,6-trimethylbenzoyldiphenylphosphine oxide(available as BASF Lucirin TPO),2,4,6-trimethylbenzoylethoxyphenylphosphine oxide (available as BASFLucirin TPO-L), bis(2,4,6-trimethylbenzoyl)-phenyl-phosphine oxide(available as Ciba IRGACURE 819) and other acyl phosphines, 2-benzyl2-dimethylamino 1-(4-morpholinophenyl) butanone-1 (available as CibaIRGACURE 369), titanocenes, and isopropylthioxanthone,1-hydroxy-cyclohexylphenylketone, benzophenone,2,4,6-trimethylbenzophenone, 4-methylbenzophenone,2-methyl-1-(4-methylthio)phenyl-2-(4-morphorlinyl)-1-propanone,diphenyl-(2,4,6-trimethylbenzoyl) phosphine oxide,2,4,6-trimethylbenzoylphenylphosphinic acid ethyl ester,oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl) propanone),2-hydroxy-2-methyl-1-phenyl-1-propanone, benzyl-dimethylketal, andmixtures thereof may be used. Amine synergists, for example such asethyl-4-dimethylaminobenzoate and 2-ethylhexyl-4-dimethylamino benzoate,may also be used. This list is not exhaustive, and any knownphotoinitiator that initiates the free radical reaction upon exposure toa desired wavelength of radiation such as UV light can be used.

The total amount of photoinitiator included in the particles withrespect to the radically curable component may be from, for example,about 0.5 to about 20%, for example preferably from about 1 to about 15%or from about 1 to about 10%, by weight.

In making the crosslinkable particles, the particles may be made thesame as any of the aforementioned emulsion aggregation methods, with themodification that the one or more crosslinking additives andphotoinitiators is included in the emulsion. The particles are thenaggregated and/or coalesced as normal. Following completion of theparticle formation, the particles may then be subjected to radiationsuch as thermal or UV radiation to initiate and effect the crosslinking.Following radiation curing, the particles still have substantially thesame size and shape, but are crosslinked and thus much more resistant tosolvents and to melting at higher temperatures.

In embodiments, one or more sets of the colored particles incorporatedinto the display medium comprise emulsion aggregation particles derivedfrom polymers having maleic anhydride and/or maleic acid functionalityincorporated into the resin. In the presence of water, the maleicanhydride groups are hydrolyzed to carboxylic acid groups (maleic acid).Depending on the mode of preparing the polymer resin used to make theparticles, the degree of hydrolysis of the maleic anhydride groups canbe altered. In the emulsion aggregation process, the introduced acidgroups permit aggregation into larger particles as well as impart asubstantially uniform negative charge to the particles. In other words,in emulsion aggregation processes, the acid functionality is used as anaggregation/coalescence site permitting larger size particles to begrown from the polymer latex. Moreover, it is believed that the acidfunctionality, for example carboxylic (COOH) acid functionality, mayimpart the substantially uniform negative charge to the particles.

An advantage in the use of these particles is that the negative chargeof the particles is substantially uniform among the particles of theset. Substantially uniform charge among the particles of a same coloredset of particles refers to, for example, a charge distribution such thatthe charge among any two given particles of the set is within about 20%,such as within about 10%, of each other. As a result, theelectrophoretic mobility of all of the particles in the set issubstantially the same, allowing the particles in the set to have asubstantially same response time upon application of an electric field.Ensuring a substantially uniform charge, and thus a substantiallyuniform mobility and response time upon application of an electric fieldis advantageous to avoid unintended mixing of one set of coloredparticles with a differently colored set of particles, for examplebecause some of the particles of the colored set did not adequatelyrespond to the electric field and permitted differently coloredparticles of a different set to integrate into the set of coloredparticles. Color degradation of the intended image could result from alack of uniformity in charge among particles of the set.

The formation of polymers having maleic anhydride functionality isdescribed in U.S. Pat. No. 7,009,011, which is incorporated herein byreference in its entirety. Specifically, any of the polymers/donormonomers, free radical initiators, stable free radical agents, optionaladditives or other components described in the above-identifiedapplication may be suitably used herein. Example polymers/donor monomersthat may be made to include maleic anhydride functionality include, forexample, styrene, butyl acrylate, carboxy ethyl acrylate, mixturesthereof, and the like.

The maleic anhydride functionality may be incorporated into the polymerat any stage of making the polymer, and the degree of conversion to themaleic acid can also be altered by the mode of preparation. For example,the maleic anhydride functionality may be introduced into the polymer ata bulk polymerization step, or at the latex formation step, which latexis used in the subsequent formation of the particles, for example byemulsion polymerization, and the like. In bulk polymerization, theprocedure is carried out in the absence of water, and the maleicanhydride functionality is left intact. When this resin is emulsifiedinto a latex, only the surface maleic anhydride groups are converted tothe acid form. Conversely, when the maleic anhydride functionality isadded to a waterborne polymer latex, all of the maleic anhydride groupsare hydrolyzed to the acid form. The particles may be made by emulsionpolymerization and the like, using the maleic anhydride functionalpolymer latex mentioned above as a starting latex, via any of theemulsion aggregation procedures discussed above.

In emulsion aggregation processes, aggregation is conducted usinglatex(es) in an aqueous medium. As a result, acid functionality, forexample carboxylic acid groups, is imparted to the particles because inmaleic anhydride hydrolyzes in the aqueous medium. Excess acidfunctionality not necessary for the aggregation procedure may providethe negative charge exhibited by the particles.

In embodiments, one or more sets, for example one to ten, such as one tofour or two to four sets, of the colored particles incorporated into thedisplay medium comprise particles, for example emulsion aggregationparticles such as emulsion aggregation polyester or emulsion aggregationacrylate particles, surface treated with a cationic polymer that impartsa substantially uniform positive charge to the particles of theparticles set. Thus, an advantage in the use of these particles is thatthe positive charge of the particles is substantially uniform among theparticles of the set. Substantially uniform charge among the particlesof a same colored set of particles refers to, for example, a chargedistribution such that the charge among any two different particles ofthe set is within about 20%, such as within about 10%, of each other. Asa result, the electrophoretic mobility of all of the particles in theset is substantially the same, allowing the particles in the set to havea substantially same response time upon application of an electricfield. Ensuring a substantially uniform charge, and thus a substantiallyuniform mobility and response time upon application of an electricfield, is advantageous to avoid unintended mixing of one set of coloredparticles with a differently colored set of particles, for examplebecause some of the particles of the colored set did not adequatelyrespond to the electric field and permitted differently coloredparticles of a different set to integrate into the set of coloredparticles. Color degradation of the intended image could result from alack of uniformity in charge among particles of the set.

In embodiments, the cationic polymer is a methacrylate polymer orcopolymer, for example an aminomethacrylate polymer such as EUDRAGIT EPO(Rohm America), that imparts a positive charge to the particles. Otherexamples of specific cationic polymers that may be selected are EUDRAGITRL and RS (Rohm Pharma), which are copolymers synthesized from acrylicand methacrylic esters with a low content of quaternary ammonium groups.EUDRAGIT RL and RS differ in the molar ratios of the ammonium groups tothe remaining neutral (meth)acrylic acid esters (1:20 and 1:40,respectively). EUDRAGIT NE is an aqueous dispersion of a neutralcopolymer based on ethyl acrylate and methyl methacrylate. EUDRAGIT RD100 is a powder form of copolymers of acrylates and methacrylates with aquaternary ammonium group in combination with sodiumcarboxymethylcellulose. Another cationic polymer is EUDRAGIT RTM E (RohmAmerica), which is a copolymer of dimethylaminoethylmethacrylate andneutral methacrylic esters.

By varying the concentration of the cationic polymer used, the degree ofcharging can be varied. For example, lower concentration of cationicpolymer means less positive charge on the particles. By creating asubstantially uniform coating of the cationic polymer on the particles,a consistent surface charge can be attained, and particle mobility isthe same for all particles. Macroscopically, the toner particles allappear to move at once, giving a faster, cleaner color transition.

The EUDRAGIT methacrylate polymers such as EUDRAGIT EPO are cationic,and are pH dependent and soluble in solutions up to pH 5. The particlesof the colored particle set may thus be surface treated with thecationic polymer by adding the cationic polymer in its dissolved form toan acidified slurry of the particles. The pH is then slowly increased toabove 5, for example to about 7 to about 2 such as about 10 to about 12,so that the cationic polymer precipitates on the surface of theparticles. The cationic polymer is believed to surface treat theparticles by forming a film around the particle's surface upon theevaporation of water. The surface of the treated particles acquires thecationic characteristics of the cationic polymer, resulting in apositive charged toner.

In further embodiments, one or more sets, for example one to ten, suchas one to four or two to four sets, of the colored particlesincorporated into the display medium comprise particles, for exampleemulsion aggregation particles such as emulsion aggregation polyester oremulsion aggregation acrylate particles having deposited thereonmultiple layers of alternating cationic and anionic layers that imparteither a substantially uniform positive charge or a substantiallyuniform, negative charge, depending on the surface layer of themulti-layer coating, to the particles of the particle set. For example,where the surface layer of the multi-layer coating is a cationicmaterial, the particles will exhibit a substantially uniform positivecharge, and where the surface layer of the multi-layer coating is ananionic material, the particles will exhibit a substantially uniformnegative charge.

As was discussed above, when emulsion aggregation particles are made,such particles will typically include anionic groups on the surfacesthereof, for example carboxylic acid groups or sodio-sulfonate groupsinherited from excess surfactant used in the process, inherited from thelatex resin, and the like. Emulsion aggregation particles thus typicallypossess the negative charge discussed above, and exhibit a negativeelectrophoretic mobility in water and in dielectric fluid. This charge,while desirable and suitable for the use of the particles in anelectrophoretic display as described above, may be non-uniform. However,the presence of anionic groups on the surfaces of the particles providessites for additional cationic and anionic materials to be built up onthe particles, and this property can be advantageously used to provide amore uniform charge among the particles.

For example, the anionic groups on the particle surface enable an ionicexchange between mobile cations on the surface with a cationic material.The result is the formation of a substantially uniform nanoscale coatingaround the toner particle surface, which coating imparts a positivecharge to the particles.

Moreover, as the cationic and anionic materials, polyelectrolytematerials may be used. In this manner, alternating layers of cationicand anionic materials may be built up. That is, following formation of alayer of cationic polyelectrolyte, ionic exchange may then be conductedbetween the ionic species of the surface cationic polyelectrolyte and ananionic polyelectrolyte to deposit a uniform nanoscale anionic coatingon the surface, which coating imparts a negative charge to theparticles.

The deposition process is conducted in an aqueous solution, whichprocess is therefore very compatible with the emulsion aggregationparticle formation processes discussed above.

It is desirable to deposit multiple alternating layers of the cationicand anionic polyelectrolyte materials. For example, the coating maycontain from 2 to about 20 total layers, such as from 2 to about 10 orfrom 2 to about 8 total layers. Each layer is approximately nanoscale inthickness, having a thickness of from about 0.1 to about 30 nm, forexample from about 0.5 to about 10 nm or from about 1 to about 3 nm.Deposition of alternating layers enables complete coverage of theparticles, which may not occur with only a single layer deposition. Thisenables the particles to have a more uniform charge density.

In general, the zeta potential (mV) achieved through deposition ofpolyelectrolytes may vary from about 5 to about 100 mV, for example fromabout 5 to about 75 mV or about 10 to about 50 mV, for cationicpolyelectrolyte surface layers, and from about −5 to about −120 mV, forexample between about −5 to about −100 mV or about −10 to about −80 mV,for anionic polyelectrolyte surface layers. In general, each particledispersed in a solution is surrounded by oppositely charged ionstypically referred to as a fixed layer. Outside the fixed layer, thereare varying compositions of ions of opposite polarities, forming acloud-like area, typically referred to as a diffuse double layer, andthe whole area is electrically neutral. When a voltage is applied to thesolution in which the particles are dispersed, particles are attractedto the electrode of the opposite polarity, accompanied by the fixedlayer and part of the diffuse double layer, or internal side of a“sliding surface.” Zeta potential is considered to be the electricpotential of this inner area including this “sliding surface.” As thiselectric potential approaches zero, particles tend to aggregate.

The deposition of multiple alternating layers also enables the creationof different charge densities among different colored particle sets. Forexample, a first particle set having a multi-layer coating in which eachlayer is comprised of the same cationic and anionic polyelectrolyteswill exhibit a certain charge density, whereas a similar particle set inwhich one or more layers of the multi-layer coating use a cationicpolyelectrolyte or an anionic polyelectrolyte different than the otherpolyelectrolytes of the multi-layer coating can exhibit a charge densitydifferent from the first particle set. The use of differentpolyelectrolytes in a multi-layer coating thus enables different chargedensities to be achieved among different particle sets. This permitsdifferent particle sets to be used in a same display medium and to becontrolled differently in view of the different charge densitiespossessed by the different particle sets. Of course, in a similarmanner, different charge densities among different particle sets mayalso be achieved through the use of entirely different cationicpolyelectrolytes and/or anionic polyelectrolytes in the making of thedifferent multi-layer coatings of the different particle sets.

In embodiments, although it is necessary to use a polyelectrolyte tobuild up the multiple layer coating, it is not necessary to use apolyelectrolyte as the surface layer of the coating. A cationic oranionic non-polyelectrolyte, for example a cationic polymer as discussedabove, may be used as the surface layer of the coating.

As the cationic polyelectrolyte, any suitable polyelectrolyte may beused. Polyelectrolyte refers to, for example, any chemical compoundcapable of ionizing when dissolved. Specific examples of cationicpolyelectrolytes include poly(diallyldimethylammonium) (PDAD) chloride:

wherein n is from, for example, about 100 to about 8,000 such as fromabout 500 to about 5,000 (PDAD(Cl) may have a weight average molecularweight of from about 50,000 to about 500,000), poly(allylamine)hydrogenchloride ((PAH)Cl):

wherein n is from, for example, about 10 to about 5,000 such as fromabout 100 to about 1,000 (PAH(Cl) may have a weight average molecularweight of about 10,000 to about 100,000), and polyethyleneimine:

wherein x and y may each independently be from 1 to about 1,000 such asfrom 1 to about 500 (polyethyleneimine may have a weight averagemolecular weight of about 200 to about 50,000). Other variants ofpolyethyleneimine can be used, such as:

or C₆H₂₁N₅, a mixture of linear and branched chains, with a weightaverage molecular weight ranging from about 1,200 to about 750,000, andwhere n may vary from about 7 to about 5,000.

As the anionic polyelectrolyte, any suitable polyelectrolyte may beused. Specific examples of anionic polyelectrolytes includepoly(styrenesulfonate) sodium salt:

wherein n is from, for example, about 10 to about 5,000 such as fromabout 100 to about 1,000 (poly(styrenesulfonate) sodium salt) may have aweight average molecular weight of about 75,000 to about 250,000),polystyrene sulfonic acid, polystyrene sulfonic acid ammonium salt,polyacrylic acid:

wherein n is from, for example, about 11 to about 75,000 such as fromabout 10 to about 60,000 (polyacrylic acid may have a weight averagemolecular weight of about 2,000 to about 5,000,000), and polyacrylicacid partial sodium salt.

An additional advantage that may be realized through the use of amultiple layer coating of alternating cationic and anionicpolyelectrolytes is that the particles may be made to more readilydisperse in the fluid of the electrophoretic display medium. Forexample, the presence of cationic and/or anionic species on the surfaceof the particles may either themselves promote dispersion of theparticles in the display medium, or may be exchanged with additionalionic species that promote such dispersion. As one example, the anion,for example a Cl ion, associated with the surface of the particles as aresult of the surface layer being a cationic polyelectrolyte, may beexchanged with a dispersion enhancing ionic species such as sodiumdioctylsulfosuccinate:

In this particular example, the resulting particles are hydrophobic.

Other dispersion enhancing species include nonionic surfactants such asSPAN 20 (sorbitan monolaurate), SPAN 60 (sorbitan monostearate), SPAN 80(sorbitan monooleate), SPAN 85 (sorbitan trioleate), mixtures thereofand the like, as well as OLOA (polyisobutylenesuccinimide), or otheranionic surfactants such as SDS (sodium dodecyl sulfate) or SDBS (sodiumdodecylbenzene sulfonate).

The resulting particles having a dispersion enhancing ionic speciesthereon may readily disperse in the display medium, for example in amedium such as ISOPAR or DOW 200 5cSt silicone oil. This is because thedispersion enhancing species compatibilizes better with the oil as aresult of being a bigger, bulkier material that is more compatible withthe oil compared to a single species such as Cl⁻.

As dyes for the colorant of the particles, examples of suitable dyesinclude Usharect Blue 86 (Direct Blue 86), available from UshantiColour; Intralite Turquoise 8GL (Direct Blue 86), available from ClassicDyestuffs; Chemictive Brilliant Red 7BH (Reactive Red 4), available fromChemiequip; Levafix Black EB, available from Bayer; Reactron Red H8B(Reactive Red 31), available from Atlas Dye-Chem; D&C Red #28 (Acid Red92), available from Warner-Jenkinson; Direct Brilliant Pink B, availablefrom Global Colors; Acid Tartrazine, available from MetrochemIndustries; Cartasol Yellow 6GF Clariant; Carta Blue 2GL, available fromClariant; and the like. Particularly preferred are solvent dyes; withinthe class of solvent dyes, spirit soluble dyes are preferred because oftheir compatibility with the ink vehicles of the present invention.Examples of suitable spirit solvent dyes include Neozapon Red 492(BASF); Orasol Red G (Ciba); Direct Brilliant Pink B (Global Colors),Aizen Spilon Red C-BH (Hodogaya Chemical); Kayanol Red 3BL (NipponKayaku); Spirit Fast Yellow 3G; Aizen Spilon Yellow C-GNH (HodogayaChemical); Cartasol Brilliant Yellow 4GF (Clariant); Pergasol Yellow CGP(Ciba); Orasol Black RLP (Ciba); Savinyl Black RLS (Clariant); MorfastBlack Conc. A (Rohm and Haas); Orasol Blue GN (Ciba); Savinyl Blue GLS(Sandoz); Luxol Fast Blue MBSN (Pylam); Sevron Blue 5GMF (ClassicDyestuffs); Basacid Blue 750 (BASF), and the like. Neozapon Black X51[C.I. Solvent Black, C.I. 12195] (BASF), Sudan Blue 670 [C.I. 61554](BASF), Sudan Yellow 146 [C.I. 12700] (BASF), and Sudan Red 462 [C.I.260501] (BASF) are preferred.

Examples of pigments that may be used as the particles herein, or thatmay be used as the colorant in polymer particles, include neat pigmentssuch as, for example, titania, barium sulfate, kaolin, zinc oxide,carbon black and the like. The pigment should be insoluble in thesuspending fluid. Additional pigments may include, for example, carbonblack such as REGAL 330 carbon black, acetylene black, lamp black,aniline black, Violet PALIOGEN Violet 5100 (BASF), PALIOGEN Violet 5890(BASF); HELIOGEN Green L8730 (BASF); LITHOL Scarlet D3700 (BASF);SUNFAST® Blue 15:4 (Sun Chemical 249-0592); Hostaperm Blue B2G-D(Clariant); Permanent Red P-F7RK; Hostaperm Violet BL (Clariant); LITHOLScarlet 4440 (BASF); Bon Red C (Dominion Color Company); ORACET Pink RF(Ciba); PALIOGEN Red 3871 K (BASF); SUNFAST® Blue 15:3 (Sun Chemical249-1284); PALIOGEN Red 3340 (BASF), SUNFAST® Carbazole Violet 23 (SunChemical 246-1670); LITHOL Fast Scarlet L4300 (BASF); Sunbrite Yellow 17(Sun Chemical 275-0023); HELIOGEN Blue L6900, L7020 (BASF); SunbriteYellow 74 (Sun Chemical 272-0558); SPECTRA PAC® C Orange 16 (SunChemical 276-3016); HELIOGEN Blue K6902, K6910 (BASF); SUNFAST® Magenta12 (Sun Chemical 228-0013); HELIOGEN Blue D6840, D7080 (BASF); SudanBlue OS (BASF); NEOPEN Blue FF4012 (BASF); PV Fast Blue B2GO1(Clariant); IRGALITE Blue BCA (Ciba); PALIOGEN Blue 6470 (BASF); SudanOrange G (Aldrich), Sudan Orange 220 (BASF); PALIOGEN Orange 3040(BASF); PALIOGEN Yellow 152, 160 (BASF); LITHOL Fast Yellow 0991 K(BASF); PALIOTOL Yellow 1840 (BASF); NOVOPERM Yellow FGL (Clariant);Lumogen Yellow D0790 (BASF); Suco-Yellow L1250 (BASF); Suco-Yellow D1355(BASF); Suco Fast Yellow D1355, D1 351 (BASF); HOSTAPERM Pink E 02(Clariant); Hansa Brilliant Yellow 5GX03 (Clariant); Permanent YellowGRL 02 (Clariant); Permanent Rubine L6B 05 (Clariant); FANAL Pink D4830(BASF); CINQUASIA Magenta (DU PONT), PALIOGEN Black L0084 (BASF);Pigment Black K801 (BASF); mixtures thereof and the like.

In polymer particles, the colorant may be included in the particles inan amount of from, for example, about 0.1 to about 75% by weight of theparticle, for example from about 1 to about 50% by weight or from about3 to about 25% by weight of the particle.

In any of the foregoing particle embodiments, the particles may alsoinclude one or more external additives on the surfaces thereof. Suchexternal additives may be applied by blending, for example with aHenschel blender. In embodiments, the external additive package mayinclude one or more of silicon dioxide or silica (SiO₂), titaniumdioxide or titania (TiO₂), titanic acid, cerium oxide, calcium or zincstearate, and the like. The particles may have an average size(diameter) of from about 5 nm to about 250 nm. Mixtures of differentlysized particles may also be used, for example a first silica having anaverage primary particle size, measured in diameter, in the range of,for example, from about 5 nm to about 50 nm, such as from about 5 nm toabout 25 nm or from about 20 nm to about 40 nm and a second silicahaving an average primary particle size, measured in diameter, in therange of, for example, from about 100 nm to about 200 nm, such as fromabout 100 nm to about 150 nm or from about 125 nm to about 145 nm. Theexternal additive particles may also be treated with a surface material.

In embodiments, the external additives may be used to impart charge tothe particles. For example, a silica particle treated withpolydimethylsiloxane (PDMS) or hexamethyldisilane (HMDS) can impart apositive charge. A titanic acid treated with isobutyl trimethoxysilanecan impart a negative charge.

The density of the particles for the display medium may be substantiallymatched to that of the suspending fluid. For example, a suspending fluidmay have a density that is “substantially matched” to the density of theparticles dispersed therein if the difference in their respectivedensities is from about zero to about 2 g/ml, for example from aboutzero to about 0.5 g/ml.

Displaying of Images

In a display medium comprising the above-described low conductivityparticle sets, the particles are first charged, for example throughapplication of an electric field thereto, for an appropriate time andwith an appropriate electric field. This field-induced or in situcharging imparts the appropriate charging characteristics to each of thesets of particles in the display medium. As will be further explainedbelow, each of the sets of particles has a substantially zero charge attime t=0. Through application of the high electric field, each set ofparticles is charged to an appropriate level. Differently coloredparticle sets may be charged to different charge levels, therebyenabling the particles of each of the different sets to have differentmobility rates within the fluid.

The field-induced or in-situ charging of the particles herein may beaccomplished by any suitable method. One such method is illustrated inFIG. 17. The device 100 of FIG. 17 includes a cell 140 in which thedisplay medium may be loaded, the cell being located between a pair ofelectrodes such as parallel-plate electrodes 150, 160. An appropriateelectric field may be generated via control generator 120 and powersupply 110, and the charging monitored by electrometer 170, whichmonitors the transient current. The reflection densitometer 130 monitorsthe change in reflectance of the display medium loaded in the cell 140as it is switched back and forth by the electric field. The reflectiondensitometer may be controlled by, for example, LabVIEW interfacesoftware and a PC 180. In embodiments, the field strength applied mayrange from about 0.05 V/μm to bout 5 V/μm, for example from about 0.25V/μm to about 3 V/μm or from about 0.5 V/μm to about 2 V/μm. The fieldmay be applied for about 0.001 seconds to about 5 hours, for examplefrom about 0.005 seconds to about 2 hours or from about 0.01 seconds toabout 1 hour or from about 1 second to about 30 minutes. The field maytake any form, and may specifically be a square waveform a triangularwaveform, a sinusoidal waveform and the like.

The charging electric field may be applied to the display fluid afterformation, that is, after addition of all of the differently coloredparticle sets thereto. Moreover, the field may be applied to the displayfluid after the display fluid is located in a multiplicity of reservoirsof the display device to form a display layer of the device, or it maybe applied to the display fluid prior to inclusion in the multiplicityof reservoirs of a display layer of the display device. If field inducedcharging is conducted on the display medium with multiple particle setstherein, the different particle sets should be chosen so as to eachcharge to a different charge level under application of a same chargingfield.

Application of different waveforms and field strengths, as well asproperties of the display medium such as size of the particles therein,surfactants used in the manufacture of the particles, the composition ofthe polymers of the particles and/or inclusion on or in the particles ofcharge agents such as discussed above, and the like, affect the chargingbehavior of the particles in the display medium. The following examplesillustrate the foregoing.

FIG. 18 shows the transient current characteristics for a display mediumcomprised of a yellow toner (Imari MF, a yellow emulsion aggregationstyrene butylacrylate toner) dispersed in ISOPAR M (the solids loadingof toner in ISOPAR M is 8% by weight) and using square-wave electricfields. FIG. 19 shows the total charge of the particles in the displaymedium acquired at different field strengths as determined from theintegrated area under a current-time curve. Note that the charge valuesidentified in the Figures refer to the total charge in the test cell innC. To calculate the charge per unit mass (in μC/g), the total charge inthe test cell is divided by the mass of toner in the test cell. Thetotal mass is derived from the ink density. Herein, a standard value of14 mg was used, which is typical for the mass of toner in an 8 wt. % inkin the cell. It can be seen from FIGS. 18 and 19 that theelectrophoretic particles become charged by the electric field, and thatcharging increases with increasing electric field strength.

FIG. 20 shows the transient current characteristics for the same displaymedium used for FIGS. 18 and 19 using a triangular-wave electric field(300 millihertz) as a function of charging cycling time. The electricfield is reported in units of V/μm, wherein μm is the gap between theelectrodes. A peak is reflected where the particles jump from one sideof the gap to the other, which temporarily peaks the current. Anelectric field peak around 1 V/μm indicates that for an electrode gap of50 μm, a 50 V field is required to effect the jump. The total charge ofthe particles is shown in FIG. 21. The results again demonstrate thatparticles are charged by the electric field and that charging increaseswith cycling time. Also, the charging may be manipulated as a result ofthe type of wave applied for charging. The ink conductivity, given bythe slope of the straight line portion of the current versus fieldcurve, is about 1.9×10⁻¹² S/m, indicating that there are very few freeions in the display medium. The electric field strength, the cyclingfrequency (waveform), and the display medium materials are theparameters which appear to most significantly influence how fast theparticles are charged. Similar results are obtained for differentlycolored particles, for example magenta, cyan and black Imari MF toners.

FIG. 22 shows an example of the charging characteristics forelectrophoretic ink particles having three different sizes (7.2 μm, 9.3μm and 16.8 μm). Each display medium is comprised of the indicated sizeof SFRP cyan styrene butylacrylate toner particles dispersed in ISOPAR M(the solids loading of toner in ISOPAR M is 8% by weight). As shown inFIG. 22, the smallest particles are able to acquire the highest charge,whereas the largest particles obtain the least charge, when charged forthe same time and using the same charging waveform.

As also can be seen in FIGS. 19, 21 and 22, the particles may be made topossess a different charge, depending on how long the particles aresubjected to the electric field. In other words, the particles mayexhibit dynamic charging characteristics wherein the charge possessed bythe particles may be ramped up where the field is applied longer and/orstronger. This enables differently colored but similarly composed andsized particle sets to be used together in a display device, since eachof the similar but differently colored particle sets may still be madeto have different charges so as to have different electrophoreticmobilities in the display device. In other words, the charge level of agiven set of colored particles in embodiments is tunable via applicationof the charging field.

FIG. 23 shows a different charging behavior. Specifically, FIG. 23 showsthe charging characteristics of an electrophoretic display mediumcomposed of a conventional cyan polyester toner dispersed in ISOPAR M.The solids loading of toner in ISOPAR M is 8% by weight. This polyestertoner is prepared via a conventional physical grinding process, not achemical process such as emulsion aggregation. The conventional processfor making polyester toner is a condensation polymerization of a diol(such as propylene glycol) and acid (such as terephthalic acid). Thebulk polymer is then mechanically pulverized via extrusion in thepresence of pigment to make fine toner particles. As can be seen in FIG.23, the charging behavior is static, that is, the particles obtainsubstantially the same charge regardless of the length of time the fieldis applied. A factor for the static charging exhibited by the polyestertoner is the absence of surfactants, coagulants, and other ionic speciesthat are present in the emulsion aggregation toner preparation process.

As was discussed above, the different particle sets included in adisplay medium nay each be made to have a different electrophoreticmobility, for example through, having a different charge. For example,in a display medium containing four differently colored particle setssuch as cyan, yellow, magenta and black, the cyan may be controlled tohave a charge of about 3 μC/g, the yellow a charge of about 2 μC/g, themagenta a charge of about 1 μC/g and the black a charge of about 0.5μC/g. The sets of differently colored particles thus should not have asubstantially similar charge level, and thus for example each particleset should have a charge differing by at least about 0.1 μC/g fromanother differently colored set of particles, for example from about 0.3μC/g or about 0.7 μC/g from each other, or more.

Under application of an appropriate AC or DC current to the displaymedium following the field induced charging, the charged particles inthe display medium having different charge levels will move at differentrates in response to the field, enabling the needed control over themovement of the particles to permit different colors to be displayed.Thus, through selection of appropriate differently colored particles,for example including the selection of particles composed of differentmaterials, made by different methods, having different sizes, havingdifferent dynamic versus static charging characteristics, and the like,and/or through control over the charging of the differently coloredparticles, a multiple color and/or full color display can be obtained byincluding differently charged, differently colored particle sets in thedisplay medium.

The field induced charging may be conducted on the display medium priorto use of the display device containing the display medium in formingimages. Also, the field induced charging procedure may be repeatedduring the lifetime of the display device in order to renew or refreshthe charges carried by the particles in the display medium. This permitsthe device to have a longer life, even where the particles in thedisplay medium exhibit charge degradation over time. Here again, becausethe particles have low conductivity and do not depend on excess freeions in the display medium for charging, the particles are able tore-charge to substantially the same levels upon reapplication of thefield induced charging field, thereby enabling the device to have alonger useful life. For this refreshing or recharging embodiment, it isagain desirable to employ display mediums with multiple particle setswherein the different particle sets each charge to a different chargelevel under application of a same electric field, so that no two sets ofdifferently colored particles are made to acquire a substantiallysimilar charge following the refreshing step.

In operating the electrophoretic display device so as to form an imagetherewith, an electric field, in particular a reversible direct currentor an alternating current, is applied to the reservoirs of the device inorder to move a desired color set of particles in the reservoirs so asto be displayed.

In embodiments of the display device, each of the individual reservoirsmay be individually addressable, that is, a separate field may beapplied to each individual reservoir of the device in order to generatean appropriate color at that individual reservoir or capsule.Appropriate sets or groups of different ones of the individualreservoirs may also be associated with a same driving electrode. Forexample, in a display, each reservoir or a set of reservoirs mayrepresent a pixel or sub-pixel of an image, and each pixel or sub-pixelmay thus be separately controlled to generate a desired overall imagefrom the device. Control methods, including hardware/software, forcontrolling each reservoir of the display device in a manner enabling anoverall image to be shown are known in the display arts, and any suchcontrol method may be applied herein. To permit individualaddressability, the size of the electrodes may be the same as or smallerthan the size of the individual reservoirs of the display device,enabling individual control of each. In this manner, the electric fieldapplied to each reservoir/capsule can be individually controlled. Also,the size of the electrodes can be different from (for example, largerthan) the size of the reservoirs, thereby enabling more than onereservoir to be controlled by a single electrode where the electrode islarger than the reservoir/capsule, or also enabling only a portion ofthe reservoir to be controlled (turned on and off) by an electrode wherethe electrode is smaller than the size of a reservoir. That is, thepattern of the electrodes does not need to line up with the reservoirs.Any of the foregoing can be done by for example, appropriate patterningof the conductive path on the bottom conductive substrate. An example ofthe patterning of electrodes can be found in, for example, U.S. Pat. No.3,668,106, incorporated herein by reference in its entirety.

Control of the color displayed by an individual reservoir of a displaydevice may be demonstrated through the following explanation. In thisexample, the display medium contains at least four differently coloredparticle sets of cyan, yellow, magenta and black, the cyan having acharge of about 3 μC/g the yellow a charge of about 2 μC/g the magenta acharge of about 1 μC/g and the black a charge of about 0.5 μC/g. As aresult of each differently colored particle set having a differentcharge, specifically a different low conductivity charge, eachdifferently colored particle set will respond differently to an appliedelectric field (that is, each differently colored particle set exhibitsa different electrophoretic mobility). In this example, the cyanparticles carry the highest charge level, and thus respond most rapidlyunder an applied electric field. Thus, to display the cyan particles toa viewer, the particles may first be pulled (attracted) to the rearsubstrate by application of an electric field. Upon reversal of theelectric field, the cyan particles will be most rapidly attracted to thefront facing electrode, such that the viewer will perceive only cyan atthat reservoir/capsule.

The set of yellow particles has the second highest charge level. Todisplay the yellow particles, the electric field from the cyan colordisplay above is again reversed to pull the particle sets back towardthe rear electrode. However, the field is applied for only so long asnecessary for the cyan particles to move past the yellow particlestoward the rear electrode. Once the cyan particles have moved past theyellow particles, the yellow color is perceived by a viewer since atthis point the yellow particles are closest to the front electrode. Ifthe reversal of the field is applied for a longer time, then the yellowparticles will move past the magenta particles toward the rearelectrode. Halting application of the field at this transition pointwill enable magenta to be perceived by the viewer since at this pointthe magenta particles will be closest to the front electrode. Finally,as the black particles in this example move slowest because they possessthe lowest charge, maintaining the reversal of the field until themagenta particles move past the black particles, for example maintainingthe reversal of the field until the particle sets in the display mediumare pulled to the back electrode, enables the black particles to beperceived by the viewer since at this point the black particles will beclosest to the front electrode.

The strength of the electric field that may be applied to effectmovement of the particles may be defined as the voltage divided by thethickness of the gap between the two electrodes. Typical units forelectric field are volts per micron (V/μm). FIG. 19 shows the chargelevel of the particle vs. the applied electric field. The electric fieldranges from 0.5 to 3 V/μm. Applied electric fields may range from about0.1 V/μm to about 25 V/μm, for example from about 0.25 V/μm to about 5V/μm, or from about 1 V/μm to about 2 V/μm, or any ranges in between.The duration of electric field application can range from about 10 msecto about 5 seconds, or from about 100 msec to about 1 second, or anyranges in between. Generally, the greater the charge on the particles,the faster the particles will move for a given electric field strength.For example, by looking at FIG. 18, the transit time is the highest peakof the curve. This transit time represents the average time for all theparticles to jump from one electrode to the other. Clearly, for the 600V curve, the transit time peak occurs at just past 0.02 sec (20 msec).Using FIG. 8 as an example, if one imagined that the various voltagecurves represented various particle groups' mobilities, at 20 msec oneset of particles (the 600 V trace) would have crossed the gap, but theother sets of particles (represented by the other traces) would be only½ or ⅓, or maybe only ¼ of the way across the gap. This information thuscan be used to determine the field strengths and application durationsnecessary to display each of the colors of a multiple color displaymedium.

Of course, any colored particle set in the display medium may be made tomove more rapidly than a differently colored particle set withoutrestriction, and the ordering of mobilities in this example is arbitraryfor illustration purposes.

As another specific example of controlling color display is amulti-color display medium, reference is made to FIGS. 34 to 27. Here,yellow particles (Y) are made to have a high positive charge and magentaparticles (M) to have a low positive charge, with cyan (C) having a highnegative charge and black (K) a low negative charge. The particles withthe higher charge are shown larger in the Figures, but this larger sizeis to depict the larger charge and not necessarily the actual sizerelationship among the particles. The particles may all have the samesize, or the larger charge particles may actually be smaller in sizethan the lower charge particles.

To enable the selective migration of the desired set of coloredparticles, the driving voltage waveform is changed from positive tonegative polarity or vice versa. When the top plate is charged + (FIG.25), the − charged pigments are attracted to this electrode. The highercharge particles, in this case cyan, will be the first particles to moveto this electrode, followed by the lower mobility black particles, andthus cyan is displayed. When the top plate potential is switched from +to − (FIG. 24), the fast moving + particles, in this case yellow, areattracted first, followed by the slower moving magenta species. Theviewing of the highly charged particles is thus relativelystraightforward, as they will be always be the first particles to reachthe oppositely charged electrode.

In order to selectively view the lower mobility species, the voltagewaveform is modified by the addition of a brief switching voltage pulseas shown in FIGS. 26 and 27. This selective pulse reverses the polarityof the current/electric field across the conductive substrates and thusreverses movement of the highly charged particles for a brief instant,and causes these particles to move toward the middle of the cell. Theelectric field is then removed once the higher mobility particles havemoved past the lower mobility particles toward the rear substrate, andbefore the additional particle sets of opposite polarity are movedcloser to the front viewing conductive substrate than the lower mobilityparticles. What remains on the outside (that is, a viewable side) arethe slow moving low mobility particles, as they are much less sensitiveto this pulsed electric field. Thus, by pulsing the electric field toattract negative charge particles to the rear substrate, the lowercharge black negative particles are displayed in place of the highernegative charge cyan particles (FIG. 27). Similarly, when the higherpositive charge yellow particles are displayed, by pulsing the electricfield to attract the positive charge particles to the rear substrate,the lower positive charge magenta particles are displayed in place ofyellow (FIG. 26).

Embodiments, the higher mobility particles may have a charge of fromabout ±1 to about ±5 μC/g for example from about ±2 to about ±3 μC/g andthe lower mobility particles a charge of from about ±0.1 to about ±1μC/g for example from about ±0.1 to about ±0.7 μC/g.

The above controls over the display of colors in a multi-color systemmay be applied to a display medium containing any number of differentlycolored particle sets, for example including two, three, four or evenmore particle sets. Highlight color particle sets, for example bluehighlight color, red highlight color, green highlight color and the likehighlight color particle sets may be included in multi-color particlesets to add additional color range capabilities to the display, and thecontrol of the colors may be effected as described above. The totalparticle sets, including highlight color particle sets, in the displaymedium thus may be five, six, seven, eight or even more.

Upon removal of the electric field, the particles may be maintained inthe selected color state through any suitable means. For example, thesets of particles may be made to have a slightly different density fromthe display fluid such that upon removal of the field, the particlesfloat to the top or bottom of the display. Because no field is applied,the particles should substantially maintain the color order at the timethe field was removed during such settling movement. Alternatively, thefluid may have a sufficiently thick viscosity to maintain the particlecolor order upon removal of the electric field. For example, a viscosityrange of 0.65 to 20 cSt, such as from about 1 to about 20 cSt or fromabout 5 to about 20 cSt, may be appropriate. To facilitate asufficiently viscous fluid, the fluid may contain a gellant, for exampleas described in U.S. Patent Publication No. 2007-0002428, incorporatedherein by reference in its entirety. The gellant acts to thicken thefluid viscosity at lower temperatures or when an electric field is notapplied, enabling images to be fixed within the reservoir/capsule. Othermethods for fixing the displayed image could come in the form of othermeans of altering the fluid viscosity. Phenomena such aselectrorheological effects (where the fluid viscosity changes upon theapplication of an electric field), magnetic field effects (where thefluid viscosity changes in response to a magnetic field), and the likecould be utilized, if desired.

Embodiments will now be further illustrated by way of the followingexamples.

EXAMPLE 1

In this example, use of emulsion aggregation particles in a two particleelectrophoretic display is demonstrated.

Preparation of negatively charged emulsion aggregation cyan particles.Cyan toner particles are prepared via aggregating dispersions of astyrene/butylacrylate/carboxylic acid terpolymer non-crosslinked resinparticles, a second cross linked copolymeric resin ofstyrene/butylacrylate/carboxylic acid with divinyl benzene, and a cyanpigment in the presence of two cationic coagulants to provide aggregateswhich are then coalesced at temperatures above the non-crosslinked resinTg to provide spherical particles. These particles are then washed (4×)with deionized water, dried, and dry-blended with an additive packagecomprising at least a silica surface treated with polydimethylsiloxane(PDMS) and having a primary particle size of about 40 nm. Anotheradditive that may be used is a titanic acid with alkyl groupfunctionality having a primary particle size of about 40 nm.

Preparation of positively charged emulsion aggregation magenta polyesterparticles. A surface treated polyester-type emulsion aggregation toneris used for the magenta particles. The surface treatment additive is thecationic methacrylate copolymer EUDRAGIT EPO. The cationic polymer isadded in its dissolved form to the acidified toner slurry. The pH isslowly increased to 10 to 12 so that the cationic polymer precipitateson the surface of the toner.

Preparation of display medium. The two colors of particles were mixedwith DOW 200 5cSt (5 centistokes) fluid, a polydimethylsiloxane polymeravailable from Dow Corning, in a 1:1 mass ratio for a solids loading ofabout 25%. Zirconia beads were added as mixing aids to evenly dispersethe mixture of particles in the fluid. No additional external chargecontrol agents were added. The ink was sandwiched between 2 parallelplates separated by a 145 μm spacer gasket. A square wave voltage of+/−200V was applied to the two plates, and the color transition wasobserved as the two toners migrated back and forth between the twoplates.

The charge of the particles enables rapid particle translation in anelectric field, and very fast response to changes in the electric field.The device may be switched at rates of about 15 to about 20 Hz or more.As a result, the electrophoretic display may be used for video display,as the device exhibits switching rates suitable for video rates, whichrequire a frame rate of up to 30 fps (standard video rate).

EXAMPLE 2

In this example, use of a silicone fluid as a fluid in a display mediumwith emulsion aggregation particles is demonstrated.

Two colors of emulsion aggregation toner particles were mixed with DOW200 5cSt fluid, in a 1:1 mass ratio for a solids loading of about 25%.Zirconia beads were added as mixing aids to evenly disperse the mixtureof toner particles in the fluid. No additional external charge controlagents were added.

The display medium was sandwiched between two parallel plates separatedby a 145 μm spacer gasket. A square wave voltage of +/−200V was appliedto the two plates, and color transition was observed as the two tonersmigrated back and forth between the two plates.

EXAMPLE 3

Incorporation of maleic anhydride into an emulsion aggregation particleat the latex step. To a bulk polymerized styrene/butylacrylate (200 ml,˜20% conversion, Mn=1,900) was added maleic anhydride (16 g). Themixture was heated to ˜50° C. until all the maleic anhydride dissolved.This was added to an aqueous solution (600 g water and sodiumdodecylbenzenesulfonate (SDBS), 16 g) and stirred for 5 minutes. Theresulting mixture was piston homogenized 3 times at 500 BAR and thentransferred to a 1 L BUCHI reactor. Pressurizing with argon and thendepressurizing (5 times) deoxygenated the latex mini-emulsion. This wasthen heated to 135° C. After 1 hour at temperature, a solution ofascorbic acid (8.5 ml of a 0.1 g/ml concentration) was added via pump atthe rate of 0.035 ml/minute. The reaction was cooled after 6 hours toafford a resin in the latex of ˜200 microns with a solids content of24.9% and Mn=9,700 and Mw=23,000.

Aggregation of latex using diamines. To a stable free radicalpolymerization latex (707 g, 23.48% solids content) was added 660 ml ofwater and pigment (cyan blue, BTD-FX-20, 47.8 g). This was stirred atroom temperature and a diamine (JEFFAMINE, D-400, 6.89 g in 100 mlwater) was added over a 10 minute period. The resulting thickenedsuspension was heated to 55° C. over a 1 hour period. The suspension wasthen basified using NaOH (concentrated) to a pH of 7.3. This wassubsequently heated to 95° C. over a 2 hour period and maintained attemperature for 5 hours. The suspension was then cooled, filtered, andwashed 5 times with water until the filtrate conductivity was less than15 microSiemens/cm². The resulting powder was resuspended in minimalwater and freeze dried to give 130 g of a 13.4 μm particle.

EXAMPLE 4

Incorporation of maleic anhydride into an emulsion aggregation particleat the bulk polymerization step. A stock solution of styrene (390 mL)and butylacrylate (110 ml) was prepared and to 400 ml was added TEMPO(3.12 g, 0.02 mole) and vazo 64 initiator (2.0 g, 0.0125 mole). This washeated under a nitrogen atmosphere to 135° C. (bath temperature) andthen added to it dropwise a solution of maleic anhydride (9.8 g) in 100ml of the styrene/butylacrylate stock solution that had beendeoxygenated using nitrogen. The addition was done over a 30 minuteperiod after which it was stirred for 5 more minutes and then cooled toafford a poly(styrene/maleic anhydride-b-styrene/butylacrylate)(Mn=4,990 with PD=1.23) solution in styrene/butylacrylate monomer.

Preparation of poly(SMA-b-S/BA) Latex. A polymer solution of the above(300 ml), styrene (117 ml), butylacrylate (33 ml) and TEMPO (0.6 g) wasadded to a solution of SDBS (36 g, 1.2 l water) and stirred for 5minutes. Then the mixture was piston homogenized once at a pressure ofabout 500 BAR and then discharged into a 2L BUCHI reactor. This washeated to 135° C. (reactor temperature) and when the reactor reachedtemperature a solution of ascorbic acid (2.4 g in 12 ml water) was addeddropwise at a rate of 0.0283 ml/minute for a total of 8.5 ml. After 6hours at reaction temperature the reactor was cooled and 1,401.3 g oflatex was discharged affording a poly(styrene/maleicanhydride-b-styrene/butylacrylate) (Mn=39,168 with a polydispersity(PD)=1.64).

Aggregation/coalescence of latex using diamine as aggregant. To theabove latex (50 ml) was added 50 ml of water and stirred at roomtemperature while adjusting the pH to ˜1.78. To this was added dropwise2.89 g of a JEFFAMINE D400 solution (20% w/w in water) at 23-25° C. andthen slowly heated up to 60° C. over ˜1 hour. The particle size grewfrom about 200 nm to 6.8 μm. The solution pH was adjusted to pH 9.04with dilute NaOH and then further heated slowly to 95° C. over thecourse of ˜1.5 hour and maintained at temperature for 1.5 hours toafford a coalesced white particle of 6.68 μm size (Mn=39,168).

EXAMPLE 5

Preparation of Positively Charged Emulsion Aggregation Polyester TonerParticles.

Comparative Example (Control): A pilot plant batch of toner comprised ofa linear sulfonated polyester resin (12% solids) (the composition of thepolyester resin consists of approximately an equimolar amount of glycolmonomers and aromatic di ester molecules), 9% carnauba wax dispersionand 6% by weight of FLEXIVERSE BLUE (Pigment Blue 15:3, BFD1121, 47.1%solids) dispersion (Sun Chemical Co.) was prepared. Aggregation of cyanpolyester toner particles was done at 58° C. in a 30-gallon stainlesssteel reactor (of which only 20 kg of the toner yield was used for benchscale studies). The agitation rate was set initially to 100 RPM. A 5%zinc acetate solution was added as the coagulant, where 60-80% of thetotal zinc acetate solution was added quickly (600 g/min for the first30 minutes) and the remainder (80-100 g/min thereafter) is added at areduced rate. The amount of zinc acetate equaled approximately 11% ofthe total resin in the emulsion. After 7 hours of aggregation, theparticle size reached 5.24 μm with a geometric standard deviation (GSD)of 1.2. Full cooling was applied and particles were sieved at 30-35° C.through a 25 μm nylon filter bag. A portion of the toner slurry waswashed in the lab three times with deionized water after mother liquorremoval, resuspended to approximately 25% by weight solids andfreeze-dried for 48 hours to give the untreated parent toner.

Example: EUDRAGIT EPO solution (1%) was prepared by dissolving 1.26 g in124.7 g of 0.3 M HNO₃; the pH of the solution was lowered to about 2 byadding 1.0 M HNO₃. Lowering the pH to 2 ensured complete solubility ofthe polymer in solution. The total percentage of EPO to toner was toequal 3% by weight of dry toner.

The above pilot plant toner was treated in the lab via a pH shiftingprocedure where EPO is soluble or insoluble in aqueous solutiondepending on the pH of the aqueous solution. A 327 g quantity of theaqueous toner suspension (12.89% by weight solids), which was separatedfrom its mother liquor, was stirred in a 1 L glass Erlenmeyer flask on astir plate at 250-300 rpm. The pH of the toner slurry was lowered from5.5 to 2.4 with 0.3 M HNO₃. The EPO solution was added drop wise to thetoner slurry and stirred for 1 hour at room temperature. After 1 hour,the pH of the toner slurry was increased to 12.2 with 1.0 M NaOH andleft to stir overnight at 300 under ambient temperature. The surfacetreated toner was then filtered and washed four times. The filtercakewas then resuspended to approximately 25% by weight solids andfreeze-dried. The pH of the filtrates was always greater than 9.5 andshowed no sign of precipitated EPO; it can be assumed that all EPOpolymer was transferred to the toner surface. The charge on theseparticles was measured to be about 0.8 μC/g.

EXAMPLE 6

Preparation of Multilayer Coating on Emulsion Aggregation Particles.

Cationic layer: 20 g of yellow emulsion aggregation polyester toner inwhich the base resin is a linear polyester containing about 3.75 mol %sulphonation, the aggregating agent is Zn(OAc)₂, and the pigment if YFDfrom Sun Chemicals, was dispersed in 920 ml deionized water bymechanical stirring. 40 wt %. NaCl solution (ca 75 ml) was added to thesolution, followed by 2 wt % poly(diallyldimethylammonium) chloride(PDAD) (25 ml) (Mw of 100-200 k). The overall solution contains 2 wt %toner in 0.25M NaCl with 0.1 wt % PDAD. The solution was mechanicallystirred for 1 hour, filtered, and the wet toner cake was then washedwith water (900 ml) for 3 times. The particles exhibit a positive zetapotential, or example of about 15 mV, in water, ISOPAR and silicone oil.

Anionic layer: The positively charged particles are redispersed in 920ml deionized water by mechanical stirring. 40 wt % NaCl solution (ca 75ml) was added to the solution, followed by 2 wt % poly(styrenesulfonate,sodium salt (PSS) (25 ml) (Mw of <100 k). The overall solution contains2 wt % toner in 0.25M NaCl with 0.1 wt % PSS. The solution wasmechanically stirred for 1 hour, filtered, and the wet toner cake wasthen washed with water (900 ml) for 3 times. The particles exhibit anegative zeta potential, for example of about −25 mV, in water, ISOPARand silicone oil.

Multilayer formation: The positive PDAD and negative PSS layers werethen deposited in alternating manner until a desired number of layerswas formed, in this case 10 total layers. Each alternating layerexhibited the aforementioned positive or negative zeta potential.

EXAMPLE 7

Preparation of Multilayer Coating on Emulsion Aggregation Particles.

Cationic layer: 10 g of cyan emulsion aggregation poly(styrene acrylate)toner with 10% crosslinked gel content was dispersed in 400 ml deionizedwater by mechanical stirring. 40 wt % NaCl solution and 2 wt % PDAD (25ml) (Mw of 100-200 k) was added to the solution. The overall solutioncomprised 0.25M NaCl and 0.1 wt % PDAD. The solution was mechanicallystirred for 1 hour, filtered, and the wet toner cake was then washedwith water (900 ml) for 3 times. The particles exhibit a positive zetapotential in water, ISOPAR and silicone oil.

Anionic layer: The positively charged particles are redispersed in 400ml deionized water by mechanical stirring. 40 wt % NaCl solution wasadded to the solution, followed by 2 wt % (PSS) (25 ml) (Mw of <100 k).The overall solution comprised 0.25M NaCl and 0.1 wt % PSS. The solutionwas mechanically stirred for 1 hour, filtered, and the wet toner cakewas then washed with water (900 ml) for 3 times. The particles exhibit anegative zeta potential in water, ISOPAR and silicone oil.

Multilayer formation: The positive PDAD and negative PSS layers werethen deposited in alternating manner until a desired number of layerswas formed, in this case 4 total layers.

EXAMPLE 8

Preparation of Highlight Color Emulsion Aggregation Toner Particles.

Preparation of crosslinked latex B. A crosslinked latex emulsioncomprised of polymer particles generated from the emulsionpolymerization of styrene, butyl acrylate and beta carboxy ethylacrylate (β-CEA) was prepared as follows. A surfactant solution of 4.08kilograms of NEOGEN™ RK (anionic emulsifier) and 78.73 kilograms ofdeionized water was prepared by mixing these components for 10 minutesin a stainless steel holding tank. The holding tank was then purged withnitrogen for 5 minutes before transferring the resulting mixture intothe above reactor. The reactor was then continuously purged withnitrogen while the contents were being stirred at 100 RPM. The reactorwas then heated up to 76° C. and held there for a period of 1 hour.

Separately, 1.24 kilograms of ammonium persulfate initiator wasdissolved in 13.12 kilograms of deionized water. Also separately,monomer emulsion was prepared in the following manner. 47.39 Kilogramsof styrene, 25.52 kilograms of butyl acrylate, 2.19 kilograms of β-CEA,2.92 kilogram of divinyl benzene (DVB) crosslinking agent, 1.75kilograms of NEOGEN™ RK (anionic surfactant), and 145.8 kilograms ofdeionized water were mixed to form an emulsion. One (1) percent of theemulsion was then slowly fed into the reactor, while the reactor wasbeing purged with nitrogen, containing the aqueous surfactant phase at76° C. to form seeds. The initiator solution was then slowly chargedinto the reactor and after 40 minutes the remainder of the emulsion wascontinuously fed in using metering pumps over a period of 3 hours.

Once all the monomer emulsion was charged into the above main reactor,the temperature was held at 76° C. for an additional 4 hours to completethe reaction. Cooling was then accomplished and the reactor temperaturewas reduced to 35° C. The product was collected into a holding tank.After drying, the resin latex onset Tg was 53.5° C. The resulting latexwas comprised of 25 percent crosslinked resin, 72.5 percent water and2.5 percent anionic surfactant. The resin had a ratio of 65:35:3 pph:4pph of styrene:butyl acrylate:β-CEA:DVB. The mean particle size of thegel latex was 50 nanometers as measured on disc centrifuge, and theresin in the latex possessed a crosslinking value of about 50 percent asmeasured by gravimetric method.

Toner preparation. Preparation of a Blue toner (PB. 15.0)—highlightblue. 310.0 Grams of the above prepared latex emulsion (Latex A) and 100grams of an aqueous blue pigment dispersion containing 36.8 grams ofBlue pigment (PB 15.0) available from Sun Chemical Corporation, having asolids loading of 54.0 percent, were simultaneously added to 500milliliters of water with high shear stirring by means of a polytron. Tothis mixture was added a 23.5 grams (grams) of polyaluminum chloride(PAC) solution containing 3.5 grams of 10 percent solids and 20 grams of0.2 molar nitric acid, over a period of 2 minute, followed by theaddition of 23.5 grams of cationic surfactant solution containing 3.5grams of the coagulant SANIZOL B™ (60 percent active ingredients) and 20grams of deionized water and blended at speed of 5,000 rpm for a periodof 2 minutes. The resulting mixture was transferred to a 2 literreaction vessel and heated at a temperature of 50° C. for 210 minuteshours resulting in aggregates of a size of 5.7 microns and a GSD of 1.22To this toner aggregate was added 150 grams of the above prepared latex(latex B) followed by stirring for an additional 30 minutes and theparticle size was found to be 5.8 and a GSD of 1.20. The pH of theresulting mixture was then adjusted from 2.6 to 7.5 with aqueous basesolution of 4 percent sodium hydroxide and allowed to stir for anadditional 15 minutes. Subsequently, the resulting mixture was heated to90° C. and retained there for a period of 1 hour where the particle sizemeasured was 5.9 microns and a GSD of 1.20, followed by the reduction ofthe pH to 4.5 with 2.5 percent nitric acid solution. The resultantmixture was then allowed to coalesce for an additional 5 hrs. Themorphology of the particles was spherical particles. The particle sizewas 6 microns with a GSD of 1.2. The reactor was then cooled down toroom temperature and the particles were washed 4 times, where the firstwash was conducted at pH of 11, followed by two washes with deionizedwater, and the last wash carried out at a pH of 4. The particles werethen dried. The charge on these particles was measured to be about 0.02to 0.15 μC/g.

EXAMPLE 9

Preparation of crosslinked emulsion aggregation particles. Following thecompletion of a standard preparation of an emulsion (a latex (colloidaldispersion in water) of very small seed particles made ofpolystyrene/butyl acrylate copolymer), the temperature is lowered toabout 60° C. and the emulsion particle swollen with a solution ofmultifunctional acrylates and photoinitiator. The multifunctionalacrylate solution consisted of 4 parts 1,6-hexanediol diacrylate(Sartomer SR 238), 4 parts trimethylolpropane triacrylate (Sartomer SR351), 2 parts pentaerythritol tetraacrylate (Sartomer SR 295), and 0.2parts BASF LUCIRIN TPO-L photoinitiator. This solution is addedgradually to the latex, which is 90 parts solids. Following aggregationand coalescence, the suspended particles are crosslinked by circulatingthe suspension by a UV light source under nitrogen, in this case a SuperMix Photochemical Reaction Vessel (Model 7868 Ace Glass) equipped withan immersion well, lamp and power source. Following radiation, theparticles are washed.

EXAMPLE 10

Polyester resin (SPAR II, a commercially available unsaturated polyesterresin available from DOW Chemical) (90 parts) is combined with themultifunctional acrylate solution identified in the prior example in thesame proportions. The mixture is then taken through the polyesteremulsion aggregation process and irradiated as in Example 9.

EXAMPLE 11

Ten parts dipentaerythritol pentaacrylate (Sartomer SR 399), 90 partsSartomer CN 959, a high viscosity (180,000 cPs) blend aliphatic urethanediacrylate and monomer diluent, 0.2 parts BASF LUCIRIN TPO-Lphotoinitiator and 3 parts surfactant are emulsified using a highpressure piston homogenizer. The emulsion is then used in aggregationand coalescence steps to produce particles. The particles are thencrosslinked as in Example 9 above.

EXAMPLE 12

Formation of a display device with a grid pattern formed onto ITO coatedglass. SU-8 cells were patterned onto ITO coated glass plates accordingto the following procedure:

spin on SU-8-25 (should give about a 30 micron film);

softbake on a leveled hotplate, 5 minutes at 115° C.,

expose resist with UV light (˜340 nm), 3 minutes at 8 mW/cm² through aphotomask;

post exposure bake on hotplate at 115° C., 5 minutes;

develop in SU-8 developer (PGMEA);

rinse with isopropanol; and

hardbake at 150° C., 5 minutes.

The display medium comprised of cyan and magenta emulsion aggregationparticles of opposite charge was sandwiched between 2 such SU-8 cells,each 27 μm thick. A square wave voltage of +/−100V was applied to thetwo plates, and the color transition was observed as the two tonersmigrated back and forth between the two plates. Successful transitionswere realized between the cyan and magenta states.

EXAMPLE 13

Preparation of display device with microencapsulated particles. Step1—microencapsulation of the display fluid. A two-particle fluid mixturewas encapsulated using the technique of complex coacervation, under highshear, provided with an overhead mixer equipped with a 3-blade impeller.40 mL of a mixture of black and white particle sets was prepared, with afinal solids loading of 15% (w/w) and a 1.5:1 ratio of black:white inDOW 200 5cSt silicone fluid. The encapsulation solution was prepared bymixing the following solutions (heated to 40° C.): 100 mL of a6.6%/(gelatin solution, 400 mL of water, and 100 mL of a 6.6% solutionof gum arabic solution in warm water. Next, the pH of the encapsulationsolution was adjusted to 4.5 via dropwise addition of dilute acetic acidsolution. The ink mixture was poured into the encapsulation bath, andallowed to cool to room temperature. The resultant capsules werecrosslinked with gluteraldehyde, washed with water, and wet-sieved toisolate the desired capsules.

Step 2—isolation and classifying of microcapsules. The capsules slurrywas wet sieved through nylon filter screens with mesh sizes of 440, 300,200, 100, and 74 μm diameter openings with vigorous shaking. The desiredsize cut was selected for coating on a substrate.

Step 3—coating of substrate/lamination of top layer. A first ITO/MYLARsubstrate was coated with a layer of PVA (3 mils gap) on the conductive(ITO) side and was air dried for 20 hours at room temperature. Next, 6 gof wet sieved capsules (<200 μm) were separated by gravitation on afilter paper from most of the water in which they were kept. Thecapsules were mixed with a solution containing 0.5 g of PVA 30%, 3 dropsof 1-octanol (defoamer) and 75 mg of glycerol (plasticizer for PVA).This capsule slurry was coated with a blade (gap was 10 mils) on top ofthe PVA layer on the first MYLAR substrate. The film was dried at roomtemperature for 20 hours. The capsules deformed during the dewateringprocess, creating a close-packed array. The film was then coated with alayer of NEOREZ (water based polyurethane glue) by using a blade and wasdried for 1 hour at room temperature and for an additional hour at 50°C. A second ITO/MYLAR substrate was coated on the ITO side with NEOREZglue with a blade (10 mils gap), then dried for 1 hour at roomtemperature and for 30 minutes at 50° C. The two substrates werelaminated together to provide the final device, which is switchablebetween black and white states.

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also,various presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art, and are also intended to beencompassed by the following claims.

1. An electrophoretic display medium, comprising at least one set ofcolored particles in a dielectric fluid, and wherein the display mediumhas an electrical conductivity of about 10⁻¹¹ to about 10⁻¹⁵ S/m.
 2. Theelectrophoretic display medium according to claim 1, wherein the displaymedium has an electrical conductivity of from about 10⁻¹² to about 10⁻¹³S/m.
 3. The electrophoretic display medium according to claim 1, whereinthe at least one set of colored particles has a charge of from about±0.1 to about ±20 μC/g.
 4. The electrophoretic display medium accordingto claim 1, wherein the display medium is substantially free of chargecontrol additives and ionic species.
 5. The electrophoretic displaymedium according to claim 1, wherein the fluid comprises from about 10%to about 95% by weight of the display medium and each set of samecolored particles comprises from about 5% to about 50% by weight of thedisplay medium.
 6. The electrophoretic display medium according to claim1, wherein the fluid comprises decane epoxide, dodecane epoxide,cyclohexyl vinyl ether, naphthalene, tetrafluorodibromoethylene,tetrachloroethylene, trifluorochloroethylene, 1,2,4-trichlorobenzene,carbon tetrachloride, decane, dodecane, tetradecane, xylene, toluene,hexane, cyclohexane, benzene, an aliphatic hydrocarbon, naphtha,octamethyl cyclosiloxane, cyclic siloxanes, poly(methyl phenylsiloxane), hexamethyldisiloxane, polydimethylsiloxane,poly(chlorotrifluoroethylene) polymer, or mixtures thereof.
 7. Theelectrophoretic display medium according to claim 1, wherein the fluidhas a viscosity of about 0.25 centistokes to about 10 centistokes atabout 23° C. to about 27° C.
 8. The electrophoretic display mediumaccording to claim 1, wherein the fluid includes a gelling agenttherein.
 9. The electrophoretic display medium according to claim 1,wherein the at least one set of colored particles comprises polyesteremulsion aggregation particles, styrene-acrylate emulsion aggregationparticles or acrylate emulsion aggregation particles.
 10. Theelectrophoretic display medium according to claim 9, wherein the atleast one set of particles has an average particle size of from about0.5 to about 25 μm, an upper geometric standard deviation (GSD) byvolume for (D84/D50) is in the range of from about 1.1 to about 1.25,and an average circularity of about 0.92 to about 0.99.
 11. Theelectrophoretic display medium according to claim 9, wherein theparticles have a core-shell structure in which the shell issubstantially free of coloring agent.
 12. The electrophoretic displaymedium according to claim 1, wherein the fluid and the at least one setof particles have densities that are within about 10% of each other. 13.The electrophoretic display medium according to claim 1, wherein thefluid includes two immiscible fluids having different densities suchthat a first immiscible fluid having a density less than that of asecond immiscible fluid lies over the second immiscible fluid, and theat least one set of colored particles has a density in between thedensities of the two immiscible fluids such that the particles rest atan interface between the two immiscible fluids.
 14. The electrophoreticdisplay medium according to claim 1, comprising one set of coloredparticles dispersed in a colored dielectric fluid.
 15. Theelectrophoretic display medium according to claim 1, wherein the atleast one set of colored particles comprises a set of black particlesand a set of white particles.
 16. The electrophoretic display mediumaccording to claim 1, wherein the at least one set of colored particlescomprises a set of cyan particles, a set of yellow particles, a set ofmagenta particles and a set of black particles.
 17. The electrophoreticdisplay medium according to claim 1, wherein the at least one set ofcolored particles comprises a set of red particles, a set of greenparticles and a set of blue particles.
 18. An electrophoretic displaydevice, comprising a multiplicity of individual reservoirs containing adisplay medium between conductive substrates, at least one of which istransparent, wherein the display medium comprises at least one set ofcolored particles in a dielectric fluid, and wherein the display mediumhas an electrical conductivity of about 10⁻¹¹ to about 10⁻¹⁵ S/m. 19.The electrophoretic display device according to claim 18, wherein thedisplay medium has an electrical conductivity of from about 10⁻¹² toabout 10⁻¹³ S/m.
 20. The electrophoretic display device according toclaim 18, wherein the at least one set of colored particles has a chargeof from about ±0.25 to about ±10 μC/g.
 21. The electrophoretic displaydevice according to claim 18, wherein the display medium issubstantially free of charge control additives and ionic species. 22.The electrophoretic display device according to claim 18, wherein the atleast one set of colored particles comprises polyester emulsionaggregation particles, styrene-acrylate emulsion aggregation particlesor acrylate emulsion aggregation particles.
 23. The electrophoreticdisplay device according to claim 22, wherein the at least one set ofparticles has an average particle size of from about 0.5 to about 25 μm,an upper geometric standard deviation (GSD) by volume for (D84/D50) isin the range of from about 1.1 to about 1.25, and an average circularityof about 0.92 to about 0.99.